qemu_snapshot: update backing store after deleting external snapshot

qemu_snapshot: update backing store after deleting external snapshot

With introduction of external snapshot revert we will have to update
backing store of qcow images not actively used be QEMU manually.
The need for this patch comes from the fact that we stop and start QEMU
process therefore after revert not all existing snapshots will be known
to that QEMU process due to reverting to non-leaf snapshot or having
multiple branches.

We need to loop over all existing snapshots and check all disks to see
if they happen to have the image we are deleting as backing store and
update them to point to the new image except for images currently used
by the running QEMU process doing the merge operation.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>