ui/clipboard: fix serial priority

ui/clipboard: fix serial priority

The incoming grab event should have a higher serial.
See also "vdagent: introduce VD_AGENT_CAP_CLIPBOARD_GRAB_SERIAL":
https://gitlab.freedesktop.org/spice/spice-protocol/-/commit/045a6978d6dbbf7046affc5c321fa8177c8cce56

This is only a relevant fix for the -display dbus, only user of that
function.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220912102455.111765-3-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>