perf(instances): a VM page does not wait for the guest agent's addresses

A running VM's page read its addresses while it rendered: the guest agent,
when its channel is connected but it does not answer, holds the page (and
a gunicorn worker) for its whole 10 s timeout, on every load. The page now
shows them once it has loaded: vm_addresses.js asks the new
instances/<pk>/addresses/ for each NIC's addresses and fills the header's
IPv4 and each network card's IPv4 and IPv6.

instances/views.py

@@ -296,7 +296,6 @@ def _instance_page(request, pk, instance, compute):
     if can_manage_console:
         console_form = ConsoleForm(instance=instance, is_superuser=request.user.is_superuser)
     console_listener_addresses = settings.QEMU_CONSOLE_LISTENER_ADDRESSES
-    ipv4_addresses = [address for net in instance.networks for address in net["ipv4"] or []]
     try:
         userinstance = UserInstance.objects.get(instance=instance, user=request.user)
     except UserInstance.DoesNotExist:
@@ -390,6 +389,18 @@ def status(request, pk):
     return JsonResponse({"status": instance.proxy.get_status()})
 
 
+def nic_addresses(request, pk):
+    """The VM's addresses by MAC, which its page shows once it has loaded
+    (vm_addresses.js): the guest agent may take its whole timeout. A host
+    that cannot be read gives none."""
+    instance = get_instance(request.user, pk)
+    try:
+        found = instance.proxy.get_addresses()
+    except libvirtError:
+        found = {}
+    return JsonResponse({mac: {"ipv4": ipv4 or [], "ipv6": ipv6 or []} for mac, (ipv4, ipv6) in found.items()})
+
+
 def _valid_ssh_target(compute):
     """The login and hostname reach ssh as a command-line argument; a compute
     whose values would inject an ssh option (an existing or bypassed row that