core: lock active snapshot during undo preview process

This patch prevents scenarios in which commands that are dependent on
the VM's snapshot are executing in parallel to undo preview command
which is changing the active snapshot of the VM.

The active snapshot of the VM is being locked when the async part of
the RestoreAllSnapshots command begins, and will be changed to OK when
the command ends.

Change-Id: Id7a2064d5130a6fa49360a8ad8e9a1004fc44da2
Bug-Url: https://bugzilla.redhat.com/868648
Signed-off-by: Arik Hadas <ahadas@redhat.com>