(1) Do a blockdev-add:

QMP> {"execute": "blockdev-add", "arguments": {"options" : {"driver": "qcow2","id": "drive-ide2-0-0", "file": {"driver": "file", "filename": "/home/kashyapc/tinker-space/qemu-upstream/scripts/qmp/backup1.qcow2" }}}}
{"return": {}}


(2) Try a blockdev-backup, to take backup of 'drive-ide0-0':

QMP> {
    "execute": "blockdev-backup", 
    "arguments": {
        "device": "drive-ide0-0-0", 
        "target": "/home/kashyapc/tinker-space/qemu-upstream/scripts/qmp/backup1.qcow2", 
        "sync": "full"
    }
}
{"error": {"class": "GenericError", "desc": "Cannot find device=/home/kashyapc/tinker-space/qemu-upstream/scripts/qmp/backup1.qcow2 nor node_name=/home/kashyapc/tinker-space/qemu-upstream/s$
ripts/qmp/backup1.qcow2"}}


(3) However, the file _does_ exist:

$ ~/tinker-space/qemu-upstream/scripts/qmp (master) $ file backup1.qcow2 
backup1.qcow2: QEMU QCOW Image (v3), 1073741824 bytes


	* * *

On the QEMU monitor:

(qemu) info block
drive-ide0-0-0 (#block127): ./cirros-0.3.3.qcow2 (qcow2)
    Cache mode:       writeback

drive-ide3-0-0 (#block322): /tmp/test3.raw (raw)
    Removable device: not locked, tray closed
    Cache mode:       writeback

drive-ide2-0-0 (#block1005): /home/kashyapc/tinker-space/qemu-upstream/scripts/qmp/backup1.qcow2 (qcow2)
    Removable device: not locked, tray closed
    Cache mode:       writeback
(qemu)