ui/vdagent: add mouse support

This patch adds support for mouse messages to the vdagent
implementation.  This can be enabled/disabled using the new
'mouse' parameter for the vdagent chardev.  Default is on.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-6-kraxel@redhat.com>