Relatively straight-forward filtering.

* src/esx/esx_vi.h (esxVI_GetNumberOfSnapshotTrees)
(esxVI_GetSnapshotTreeNames): Add parameter.
* src/esx/esx_vi.c (esxVI_GetNumberOfSnapshotTrees)
(esxVI_GetSnapshotTreeNames): Allow leaf filtering.
* src/esx/esx_driver.c (esxDomainSnapshotNum)
(esxDomainSnapshotListNames, esxDomainSnapshotNumChildren)
(esxDomainSnapshotListChildrenNames): Pass new flag through.