qemu_snapshot: fix memory leak when reverting external snapshot

The code cleaning up virStorageSource doesn't free data allocated by
virStorageSourceInit() so we need to call virStorageSourceDeinit()
explicitly.

Fixes: 8e664737
Resolves: https://issues.redhat.com/browse/RHEL-33044


Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>