pci.lua: Code cleanup.

selftest now looks like this:

    $ sudo ./snabbswitch -e 'require("pci").selftest()'
    make: Nothing to be done for `all'.
    selftest: pci
    pciaddress    vendor  device  interface  status    driver      usable
    0000:83:00.0  0x8086  0x10fb  em1        up        intel10g    no
    0000:83:00.1  0x8086  0x10fb  -          -         intel10g    yes