Commit 9f5e53e2 introduced the ability to filter snapshots to
just roots, but it was never implemented for ESX until now.

* src/esx/esx_vi.h (esxVI_GetNumberOfSnapshotTrees)
(esxVI_GetSnapshotTreeNames): Add parameter.
* src/esx/esx_vi.c (esxVI_GetNumberOfSnapshotTrees)
(esxVI_GetSnapshotTreeNames): Allow choice of recursion or not.
* src/esx/esx_driver.c (esxDomainSnapshotNum)
(esxDomainSnapshotListNames): Use it to limit to roots.