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.

computes/forms.py

@@ -50,4 +50,4 @@ class SocketComputeForm(forms.ModelForm):
 
     class Meta:
         model = Compute
-        fields = ["name", "details", "hostname", "type"]
+        fields = ["name", "details", "hostname", "type", "migration_address"]