core: treat absence of libvirt domain as 'Halted' state

If the domain isn't defined in libvirt, it surely isn't running. This is
needed for DispVM, which compares with exactly this state.

core-modules/000QubesVm.py

@@ -802,7 +802,7 @@ class QubesVm(object):
                 return 'Halted'
         except libvirt.libvirtError:
             if vmm.libvirt_conn.virConnGetLastError()[0] == libvirt.VIR_ERR_NO_DOMAIN:
-                return "NA"
+                return "Halted"
             else:
                 raise