virtio-input: add a virtio-mulitouch device

Add a virtio-multitouch device to the family of devices emulated by
virtio-input implementing the Multi-touch protocol as descripted here:

https://www.kernel.org/doc/html/latest/input/multi-touch-protocol.html?highlight=multi+touch



This patch just add the device itself, without connecting it to any
backends. The following patches will add a PCI-based multitouch device,
some helpers in "ui" and will enable the GTK3 backend to transpose
multi-touch events from the host to the guest.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230526112925.38794-4-slp@redhat.com>