core: minor refactoring in stop vm related commands

1. replace canDoAction methods in ShutdownVmCommand and StopVmCommand
that were not doing anything besides setting the VAR__ACTION and
VAR__TYPE with setActionMessageParameters methods

2. rename StopVmCommandBase#privateSuspendedVm to suspendedVm

3. reorganize StopVmCommandBase#canDoAction method to be more readable

4. extract the checks that determine whether the VM can be shutdown from
the Perform method to an external method.

Change-Id: I8af061d2805a85dadd84efa1cd2a0a290dbddbd1
Signed-off-by: Arik Hadas <ahadas@redhat.com>