qemu_snapshot: fix snapshot deletion that had multiple children

qemu_snapshot: fix snapshot deletion that had multiple children

When we revert to non-leaf snapshot and create new branch or branches
the overlay in snapshot metadata is no longer usable as a disk source
for deletion of that snapshot. We need to use other places to figure out
the correct storage source.

Fixes: https://gitlab.com/libvirt/libvirt/-/issues/534
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>