qemu_snapshot: update metadata when deleting snapshots

qemu_snapshot: update metadata when deleting snapshots

With external snapshots we need to modify the metadata bit more then
what is required for internal snapshots. Mainly the storage source
location changes with every external snapshot.

This means that if we delete non-leaf snapshot we need to update all
children snapshots and modify the disk sources for all affected disks.

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