=============================================================
root@images$ qemu-img convert -f raw -O qcow2 -o preallocation=metadata f17test.img f17test.qcow2

root@images$ ls -lash f17test.qcow2 
1.4G -rw-r--r--. 1 root root 11G May 13 23:05 f17test.qcow2

root@images$ ls -l f17test.qcow2 
-rw-r--r--. 1 root root 10739318784 May 13 23:05 f17test.qcow2
root@images$

root@images$ fallocate -l 10739318784 f17test.qcow2 

root@images$ ls -lash f17test.qcow2 
11G -rw-r--r--. 1 root root 11G May 13 23:05 f17test.qcow2
root@images$ 
=============================================================

=============================================================
root@~$ virsh list
 Id    Name                           State
----------------------------------------------------
 11    f17test                        running

root@~$ 
=============================================================

root@~$ virsh domblklist f17test --details
Type       Device     Target     Source
------------------------------------------------
file       disk       vda        /var/lib/libvirt/images/f17test.qcow2

root@~$ 
=============================================================

root@~$ virsh snapshot-list f17test
 Name                 Creation Time             State
------------------------------------------------------------

root@~$ 
=============================================================
root@~$ virsh snapshot-create-as f17test cleanf17 cleanf17-box --diskspec vda,file=/var/lib/libvirt/images/snap-f17test.qcow2 --disk-only
Domain snapshot cleanf17 created
root@~$ 
=============================================================
root@~$ virsh snapshot-list f17test
 Name                 Creation Time             State
------------------------------------------------------------
 cleanf17             2012-05-14 00:02:54 +0530 disk-snapshot

root@~$ 
=============================================================

virsh # snapshot-dumpxml f17test cleanf17 --security-info
<domainsnapshot>
  <name>cleanf17</name>
  <description>cleanf17-box</description>
  <state>disk-snapshot</state>
  <creationTime>1336933974</creationTime>
  <disks>
    <disk name='vda' snapshot='external'>
      <driver type='qcow2'/>
      <source file='/var/lib/libvirt/images/snap-f17test.qcow2'/>
    </disk>
  </disks>
  <domain type='kvm'>
    <name>f17test</name>
    <uuid>027ffca8-00b8-3cff-b8f6-bf8c4ec655a7</uuid>
    <memory unit='KiB'>2097152</memory>
    <currentMemory unit='KiB'>2097152</currentMemory>
    <vcpu>2</vcpu>
    <os>
      <type arch='x86_64' machine='pc-0.15'>hvm</type>
      <boot dev='hd'/>
    </os>
    <features>
      <acpi/>
      <apic/>
      <pae/>
    </features>
    <clock offset='utc'/>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>restart</on_crash>
    <devices>
      <emulator>/usr/bin/qemu-kvm</emulator>
      <disk type='file' device='disk'>
        <driver name='qemu' type='qcow2'/>
        <source file='/var/lib/libvirt/images/f17test.qcow2'/>
        <target dev='vda' bus='virtio'/>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
      </disk>
      <controller type='usb' index='0'>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
      </controller>
      <controller type='ide' index='0'>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
      </controller>
      <interface type='bridge'>
        <mac address='52:54:00:98:e3:16'/>
        <source bridge='virbr0'/>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
      </interface>
      <serial type='pty'>
        <target port='0'/>
      </serial>
      <console type='pty'>
        <target type='serial' port='0'/>
      </console>
      <memballoon model='virtio'>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
      </memballoon>
    </devices>
    <seclabel type='dynamic' relabel='yes'/>
  </domain>
</domainsnapshot>