core: minor cleanup in MigrateVmToServerCommand

- remove var_action and var_type settings which are already set in
  MigrateVmCommand#setActionMessageParameters
- call failCanDoAction instead of having nested if-else statements
- move comment that explains a method to be right before the method
  instead of inside the method body

Change-Id: Ic455ae7ba1a38cd8d7e67e4d925d2ce18493227a
Signed-off-by: Arik Hadas <ahadas@redhat.com>