argo: Remove reachable ASSERT_UNREACHABLE

I observed this ASSERT_UNREACHABLE in partner_rings_remove consistently
trip.  It was in OpenXT with the viptables patch applied.

dom10 shuts down.
dom7 is REJECTED sending to dom10.
dom7 shuts down and this ASSERT trips for dom10.

The argo_send_info has a domid, but there is no refcount taken on
the domain.  Therefore it's not appropriate to ASSERT that the domain
can be looked up via domid.  Replace with a debug message.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Christopher Clark <christopher.w.clark@gmail.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/common/argo.c

@@ -1298,7 +1298,8 @@ partner_rings_remove(struct domain *src_d)
                     ASSERT_UNREACHABLE();
             }
             else
-                ASSERT_UNREACHABLE();
+                argo_dprintk("%pd has entry for stale partner d%u\n",
+                             src_d, send_info->id.domain_id);
 
             if ( dst_d )
                 rcu_unlock_domain(dst_d);