Reject user ports that have MACs the hypervisor cannot use.

If a hypervisor has requested specific MAC addresses, ports requested
by the user have to be in that set of MAC addresses, or they cannot be
used. Raising an error at network allocation time is cleaner than
waiting for the hypervisor to attempt to plug its vifs and error.

Change-Id: If556cc518a01df658044e3ba6b9d7584bfabe588

nova/exception.py

@@ -522,6 +522,10 @@ class PortNotFound(NotFound):
     message = _("Port %(port_id)s could not be found.")
 
 
+class PortNotUsable(NovaException):
+    message = _("Port %(port_id)s not usable for instance %(instance)s.")
+
+
 class FixedIpNotFound(NotFound):
     message = _("No fixed IP associated with id %(id)s.")