(1) List the block device associated with a guest. 
                ======================
                # virsh domblklist f17-base
                Target     Source
                ---------------------------------------------
                vda        /export/vmimages/f17-base.qcow2

                #
                ======================

        (2) Create external disk-only snapshot (while the guest is running).
                =====================
                # virsh snapshot-create-as --domain f17-base snap1 snap1-desc \
                --disk-only --diskspec vda,snapshot=external,file=/export/vmimages/sn1-of-f17-base.qcow2 \
                --atomic
                Domain snapshot snap1 created
                #
                ======================

        (3) Now, list the block device associated(use cmd from step-1, above)
            with the guest,again, to ensure it reflects the new overlay image as
            the current block device in use. 
                ======================
                # virsh domblklist f17-base
                Target     Source
                ----------------------------------------------------
                vda        /export/vmimages/sn1-of-f17-base.qcow2

                #
                ======================

        (4) Here, backup the orignial disk image /export/vmimages/f17-base.qcow2
            using rsync or any other preferred method, while the guest is now 
            writing all its new changes into sn1-of-f17-base.qcow2.
        
        (5) Now, let's say, there are changes accumulated over time into
            sn1-of-f17-base.qcow2. We can now merge the contents from orignial disk
            image(f17-base) into the current disk using 'blockpull'. Two possible here:
            
            (a) Say, we had this chain:  [base] <-- [sn1] (active)

                 We can make sn1-of-f17-base.qcow2 a standalone image by
                 pulling/merging contents from the orignial disk image
                 (f17-base.qcow2). Resulting in sn1-of-f17-base becoming a self-contained
                 disk image without any backing file.

                 ======================
                 # virsh blockpull --domain f17-base --path \
                 /var/lib/libvirt/images/sn1-of-f17-base.qcow2 --verbose --wait
                 ======================

                 The resultant chain will be: [sn1] 
                 (which is an active, self-contained image with all the data
                 from [base] pulled into it)

            (b) If we have more snapshots, say if we had the below image chain: 

                [base] <-- [sn1] <-- [sn2] <-- [active],

                we can shorten it to:

                [base] <-- [sn1] <-- [active]

                by doing:
                ======================
                # virsh blockpull --domain f17-base --path \
                /var/lib/libvirt/images/active.qcow2 --base /var/lib/libvirt/sn1.qcow2 --verbose --wait
                ======================
                The above command pulls the data from [sn2] into [active], and changes the backing file of [active] to [sn1].

                
                We can again do 'blockpull' here, with the above shortened chain, 
                resulting in  making [active] standalone, with the below command:
                ======================
                # virsh blockpull --domain f17-base --path \
                /var/lib/libvirt/images/active.qcow2 --verbose --wait
                ======================
                The above command pulls data from [sn1] and [base] 
                into [active] and makes it a self-contained image without any backing file,
                resulting in the chain with a single image:  [active] 
               
            
                NOTE: 'blockpull' needs to be done for every disk image that was
                snapshotted (by optionally supplying --base. If a --base option
                is not supplied, the current disk image will pull data from its
                entire backing chain, & makes it standalone without any backing
                file.)

        (6) Once the 'blockpull' operation above is complete, we can clean-up
            the tracking of snapshots by libvirt to reflect the new reality:
                
                ======================
                # virsh snapshot-delete $dom $name --metadata
                ======================