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.

Bug-Url: https://bugzilla.redhat.com/1018554
Change-Id: Ic2c0dddb778b85250f3eef451bb1bd996c9bff67
Signed-off-by: Ori Liel <oliel@redhat.com>