core: Extract method of snapshot id.

Extracting a new method to get the snapshot id instead of
working directly with the parameters command.

Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Related to Bug-Url: https://bugzilla.redhat.com/867339
Change-Id: I4754e381d70f08a9650596a4eeb3de296b3cb44f
core: Removing the use of getDisks from RestoreCommand

There is no place that initializes the disks in the
parameter class when RestoreAllSnapshot is called.
Since there is no use of it whatsoever, this patch removes the use of
it in the command, so we can decouple RestoreAllSnapshotsParameters from
TryBackToAllSnapshotsOfVmParameters

Change-Id: I1d0f5f92288ea92c114526e0d7d567a7ece45fbd
Related to Bug-Url: https://bugzilla.redhat.com/867339
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
engine: Add action type for snapshot.

The following patch changes the API between the client and the engine
when preview/commit or undo a snapshot.
The patch introduces the following changes:
* Adding an enum which distinguishes between several operations on a snapshot:
  Undo
  Commit
  Restore From Stateless status
* Use of snapshot instead snapshotId
* Changing the client to call to the engine with the operation on snapshot
* Removing snapshotId from the constructor of the parameters, since the
snapshot is only on a VM, and there is no reason to transfer it, but
better that the engine will fetch it, it self.
The reason for this change, is for REST operations to be implemented in
the following patches

Change-Id:If877befc5058c3412ae3d3731d5beacbc09e5c97
Related to Bug-Url: https://bugzilla.redhat.com/867339
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>