The two new APIs are rather trivial; based on bits and pieces of
other existing APIs.  But rather than blindly return 0 or 1 for
HasMetadata, I chose to first validate that the snapshot in
question in fact exists.

* src/esx/esx_driver.c (esxDomainSnapshotIsCurrent)
(esxDomainSnapshotHasMetadata): New functions.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotIsCurrent)
(vboxDomainSnapshotHasMetadata): Likewise.