block/nbd: call socket_address_parse_named_fd() in advance

Detecting monitor by current coroutine works bad when we are not in
coroutine context. And that's exactly so in nbd reconnect code, where
qio_channel_socket_connect_sync() is called from thread.

Monitor is needed only to parse named file descriptor. So, let's just
parse it during nbd_open(), so that all further users of s->saddr don't
need to access monitor.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610100802.5888-7-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd.c

@@ -2140,6 +2140,12 @@ static SocketAddress *nbd_config(BDRVNBDState *s, QDict *options,
         goto done;
     }
 
+    if (socket_address_parse_named_fd(saddr, errp) < 0) {
+        qapi_free_SocketAddress(saddr);
+        saddr = NULL;
+        goto done;
+    }
+
 done:
     qobject_unref(addr);
     visit_free(iv);