No need to request the parent of a snapshot if we aren't going to use it.

* src/esx/esx_vi.c (esxVI_GetSnapshotTreeByName): Make parent
optional.
* src/esx/esx_driver.c (esxDomainSnapshotCreateXML)
(esxDomainSnapshotLookupByName, esxDomainRevertToSnapshot)
(esxDomainSnapshotDelete): Simplify accordingly.