The current type for the busId options are too relaxed, a stricter
constraint should be imposed to guard against typos which result
in Xorg unable to start.
This commit restricts the type to adhere to the B/D/F notation[1] for
addressing devices as expected by the module option.
[1] - https://wiki.osdev.org/PCI#Configuration_Space_Access_Mechanism_.231
614b54d9b0