The QEMU 'tablet' input device reports absolute positions instead
of relative mouse pointer movements. This can be used to automatically
grab/release the mouse pointer when entering/leaving the QEMU window,
instead of having to release it with CTRL + ALT + G manually.
This is quite convenient and should be the default option normally,
but at least on my PC the mouse pointer is reported with some vertical
offset for some reason (you can't reach the top and it extends below
the QEMU window...). Let's add it as an optional --tablet option for now.
f602df0140