Do not check for object existence when got it with another Admin API call

When qubesd returns an name of VM or other object, as part of another
call (reading a property, listing devices etc), it's safe to assume that
object exists. Do not try to list it, which could be prevented by qrexec
policy. This means a VM object would be returned (for example in
vm.netvm property), which potentially could not be listed through
app.domains collection. This may lead to some corner cases, but
generally should ease handling of restricted policy.

This does not affect practical information the management VM have access
too, as those names are already returned. It's just client side python
wrapper that didn't allowed to access them.

QubesOS/qubes-issues#5099

qubesadmin/base.py

@@ -233,7 +233,7 @@ class PropertyHolder(object):
         if prop_type == 'vm':
             if value == '':
                 return None
-            return self.app.domains[value]
+            return self.app.domains.get_blind(value)
         if prop_type == 'label':
             if value == '':
                 return None

qubesadmin/devices.py

@@ -209,7 +209,7 @@ class DeviceCollection(object):
                               ['True', 'yes', True])
             if persistent is not None and dev_persistent != persistent:
                 continue
-            backend_domain = self._vm.app.domains[backend_domain]
+            backend_domain = self._vm.app.domains.get_blind(backend_domain)
             yield DeviceAssignment(backend_domain, ident, options,
                                    persistent=dev_persistent,
                                    frontend_domain=self._vm,