vmd(8): scan pci bus to determine bootorder strings.

vmd(8): scan pci bus to determine bootorder strings.

vmd's SeaBIOS bootorder strings had hardcoded pci device ids, so
if a user added a network interface the bootorder strings didn't
line up with reality. Using vmctl(8) to boot from a cdrom (-B cdrom)
would fail, for instance, if attaching both a nic and a disk as
well.

This change scans the pci devices and finds the first of each type
to construct viable bootorder strings.

ok jan@