refactor(instances): one way to reach a VM, its console and its devices

The .vv console file is built from the VM found by its UUID, with the
console page's getters, instead of looking the VM up by name five times;
a console without a listen address or password now gives 127.0.0.1 and
an empty password instead of "None". Removed methods nothing calls: the
by-name getters and power methods of wvmInstances, managed save, the vCPU
mode getter (it read the wrong attribute), the old NIC editor and its
bridge lookup. CD-ROM media and NIC hotplug use _definitions() like the
other device edits, the disk editor takes the bus prefix from
util.vol_dev_type, and an unreadable disk is logged, not printed.

webvirtcloud/test_authorization_matrix.py

@@ -250,7 +250,7 @@ class AuthorizationMatrixTestCase(TestCase):
         # Mock libvirt so the views would succeed if authorization let them through.
         with patch("datasource.views.wvmInstance", MagicMock()), patch(
             "datasource.views.get_hostname_by_ip", return_value="host"
-        ), patch("instances.views.wvmInstances", MagicMock()):
+        ), patch("instances.models.wvmInstance", MagicMock()):
             for name in CONSOLE_ENDPOINTS:
                 url = self._url(name, params[name])
                 response = self._request(self.viewer, url)