core: call ACPI S3 emulation only for VMs with PCI devices

Actually it looks to be needed only there.
But also another problem: this suspend doesn't work for firewallvm, for
unknown reason.

core-modules/000QubesVm.py

@@ -1542,6 +1542,7 @@ class QubesVm(object):
         if not self.is_running() and not self.is_paused():
             raise QubesException ("VM already stopped!")
 
+        if len (self.pcidevs) > 0:
             xs_path = '/local/domain/%d/control/shutdown' % self.get_xid()
             xs.write('', xs_path, 'suspend')
             tries = 0
@@ -1553,6 +1554,8 @@ class QubesVm(object):
                     self.pause()
                     break
                 time.sleep(0.2)
+        else:
+            self.pause()
 
     def resume(self):
         if dry_run: