feat(computes): a compute names the address migrations reach it on

Migration is client-driven: libvirt sends the VM's memory to the
destination's own host name unless a migration URI is given, and the
app never gave one. Where the hosts cannot resolve each other's names
(or the names point at a management network) every migration failed
with "Unable to resolve address '<host name>' service '49152'".

A compute now has an optional migration address (an IP address or a
host name, no scheme or port), editable in the compute form and the
API. A VM migrating to that compute is sent to tcp://<address>
(bracketed for IPv6). Empty, the default for existing computes, keeps
the previous behavior. The compute's host name is not reused, because
for SSH computes it can be an ssh_config alias.

The migration log entry names the address when one is set.

instances/utils.py

@@ -191,6 +191,7 @@ def migrate_instance(
                     autoconverge,
                     compress,
                     postcopy,
+                    uri=new_compute.migration_uri,
                 )
             finally:
                 if conn_migrate is not None:

instances/views.py

@@ -687,6 +687,8 @@ def migrate(request, pk):
             "hostname": target_host.hostname,
             "method": migration_method,
         }
+        if target_host.migration_address:
+            msg = _("%(msg)s via %(address)s") % {"msg": msg, "address": target_host.migration_address}
         addlogmsg(request.user.username, current_host, instance.name, msg)
 
     return get_safe_redirect(