fix(instances): an empty panel says what to do next

- The VM list with no VMs no longer also says that no VM matches the
  search and the state.
- The New VM dialog with no compute yet says to add the first host, with a
  link to the Computes page (and links there when none is reachable).
- The new VM page on a host without a running storage pool or a network
  says what to add or start and where, instead of "You haven't defined any
  storage pools" (also when pools exist but are stopped) and "network
  pools".

instances/tests/test_create_form_input.py

@@ -61,7 +61,7 @@ class CreateFormInputTests(TestCase):
             _, response = render_create_page(self)
         self.assertIsNone(response.context["vm_form"])
         self.assertEqual(html.fromstring(response.content).xpath("//button[@name='create']"), [])
-        self.assertTrue(any("storage pools" in str(m) for m in response.context["messages"]))
+        self.assertTrue(any("no running storage pool" in str(m) for m in response.context["messages"]))
 
     def test_a_host_that_cannot_be_read_is_a_message(self):
         import libvirt