nbd: Forbid nbd-server-stop when server is not running

Since we already forbid other nbd-server commands when not
in the right state, it is unlikely that any caller was relying
on a second stop to behave as a silent no-op.  Update iotest
223 to show the improved behavior.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20190111194720.15671-3-eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

blockdev-nbd.c

@@ -214,6 +214,11 @@ void qmp_nbd_server_remove(const char *name,
 
 void qmp_nbd_server_stop(Error **errp)
 {
+    if (!nbd_server) {
+        error_setg(errp, "NBD server not running");
+        return;
+    }
+
     nbd_export_close_all();
 
     nbd_server_free(nbd_server);

tests/qemu-iotests/223

@@ -172,7 +172,7 @@ _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-remove",
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-remove",
   "arguments":{"name":"n2"}}' "error" # Attempt duplicate clean
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-stop"}' "return"
-_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-stop"}' "return" # Oops
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-stop"}' "error" # Again
 _send_qemu_cmd $QEMU_HANDLE '{"execute":"quit"}' "return"
 
 # success, all done

tests/qemu-iotests/223.out

@@ -69,6 +69,6 @@ read 2097152/2097152 bytes at offset 2097152
 {"return": {}}
 {"error": {"class": "GenericError", "desc": "Export 'n2' is not found"}}
 {"return": {}}
-{"return": {}}
+{"error": {"class": "GenericError", "desc": "NBD server not running"}}
 {"return": {}}
 *** done