I tried to launch Minix3 in Qubes OS. While there is no problem to boot
minix as a qube (in Qubes OS terminology) before 3641562, it fails with
the commit (and after). I didn't digg into PCI handling but this change
fixes the problem. Minix handles NULL case from pci_subclass_name.
Change-Id: I162424d92b613598e6eb845a71f90a02e31041db