hw/i386/vmmouse:add relative packet flag for button status

hw/i386/vmmouse:add relative packet flag for button status

The buttons value use macros instead of direct numbers.

If request relative mode, have to add this for
guest vmmouse driver to judge this is a relative packet.
otherwise,vmmouse driver will not match
the condition 'status & VMMOUSE_RELATIVE_PACKET',
and can't report events on the correct(relative) input device,
result to relative mode unuseful.

Signed-off-by: Zongmin Zhou<zhouzongmin@kylinos.cn>
Message-ID: <20230413081526.2229916-1-zhouzongmin@kylinos.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>