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/validators.py

@@ -1,4 +1,5 @@
 import re
+from ipaddress import ip_address
 
 from django.core.exceptions import ValidationError
 from django.utils.translation import gettext_lazy as _
@@ -24,3 +25,17 @@ def validate_name(value):
     have_symbol = wrong_name.match("[^a-zA-Z0-9._-]+")
     if have_symbol:
         raise ValidationError(_("The hostname must not contain any special characters"))
+
+
+_hostname = re.compile(r"[A-Za-z0-9]([A-Za-z0-9-]{0,61}[A-Za-z0-9])?(\.[A-Za-z0-9]([A-Za-z0-9-]{0,61}[A-Za-z0-9])?)*\.?")
+
+
+def validate_migration_address(value):
+    """An IP address (no IPv6 scope) or a host name: no scheme, port or brackets."""
+    try:
+        if not getattr(ip_address(value), "scope_id", None):
+            return
+    except ValueError:
+        if _hostname.fullmatch(value):
+            return
+    raise ValidationError(_("Enter an IP address or a host name, without a scheme or port"))