restapi : Reverting Missing opertions for snapshots in RESTAPI(#1018554)

Reverting vm snapshot undo, revert and commit operations
in RESTAPI as there is no supporting implementation in
the backend.

Change-Id: Ie4c23c4f0032acd90405b2af8fdff8f32d65bee1
Bug-Url: https://bugzilla.redhat.com/1018554
Signed-off-by: Ravi Nori <rnori@redhat.com>