xfree86: On Linux, while only seat0 can have TTYs, don't assume all seat0s have TTYs
I wasn't sure the best place to put +xf86HasTTYs
to where I could call it from hw/xfree86/os-support/linux/systemd-logind.c and hw/xfree86/common/xf86Init.c and the most sensible place to be able to call major()