dom0: qvm-run --nogui support for HVM

Translate it to "nogui:" command prefix.

dom0/qvm-core/qubes.py

@@ -2406,6 +2406,8 @@ class QubesHVm(QubesVm):
 
     def run(self, command, **kwargs):
         if self.qrexec_installed:
+            if 'gui' in kwargs and kwargs['gui']==False:
+                command = "nogui:" + command
             super(QubesHVm, self).run(command, **kwargs)
         else:
             raise QubesException("Needs qrexec agent installed in VM to use this function. See also qvm-prefs.")