qubesvm: Let HVMs enter 'Running' state

Prior to this commit, a properly configured Linux HVM would not
transition from the 'Transient' state to the 'Running' state according
to qvm-ls output, even if the HVM in question had the 'qrexec' feature
disabled.

This issue is caused by an unconditional qrexec check in the
'on_domain_is_fully_usable' method, and is resolved by adding
a check that short-circuits the qrexec check if the aforementioned
feature is not enabled for the VM in question.

qubes/vm/qubesvm.py

@@ -1953,9 +1953,11 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
         Currently this checks for running qrexec.
         """  # pylint: disable=unused-argument
 
-        # Running gui-daemon implies also VM running
-        if not self.is_qrexec_running():
-            yield False
+        if self.features.check_with_template('qrexec', False):
+            # Running gui-daemon implies also VM running
+            yield self.is_qrexec_running()
+        else:
+            yield True
 
     # memory and disk