overqemu: Add "-M" to disable the tablet pointing device

I couldn't get the mouse to work in OpenBSD. /dev/wsmouse simply remains
silent, nothing to read.

So, "-M" is more like a workaround. That's how I get a usable X on
OpenBSD:

	$ overqemu -M -d sdl -b ~/VMs/openbsd55.qcow2