ui/vdagent: add clipboard support

This patch adds support for clipboard messages to the qemu vdagent
implementation, which allows the guest exchange clipboard data with
qemu.  Clipboard support can be enabled/disabled using the new
'clipboard' parameter for the vdagent chardev.  Default is off.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-7-kraxel@redhat.com>