qemu: Reparent children when deleting a snapshot

qemu: Reparent children when deleting a snapshot

Shorten qemuDomainSnapshotWriteSnapshotMetadata function name
and make it take a snapshot pointer instead of dealing with
the current snapshot. Update other functions accordingly.

Add a qemuDomainSnapshotReparentChildren hash iterator to
reparent the children of a snapshot that is being deleted. Use
qemuDomainSnapshotWriteMetadata to write updated metadata
to disk.

This fixes a problem where outdated parent information breaks
the snapshot tree and hinders the deletion of child snapshots.

Reported by Philipp Hahn.