app: fix get_free_xen_memory function

It's app.vmm.xc, not app.xc. Since nobody have noticed it, this function
might be unused...

qubes/app.py

@@ -309,7 +309,7 @@ class QubesHost:
         :raises NotImplementedError: when not under Xen
         """
         try:
-            self._physinfo = self.app.xc.physinfo()
+            self._physinfo = self.app.vmm.xc.physinfo()
         except AttributeError:
             raise NotImplementedError('This function requires Xen hypervisor')
         return int(self._physinfo['free_memory'])