Changes required for OpenBSD/amd64:

Changes required for OpenBSD/amd64:

- Allow a hostbridge to be created with AMD as a vendor.
  This passes the OpenBSD check to allow the use of MSI
  on a PCI bus.
- Enable the i/o interrupt section of the mptable, and
  populate it with unity ISA mappings. This allows the
  'legacy' IRQ mappings of the PCI serial port to be
  set up. Delete unused print routine that was obscuring code.
- Use the '-W' option to enable virtio single-vector MSI
  rather than an environment variable. Update the virtio
  net/block drivers to query this flag when setting up
  interrupts.: bhyverun.c
- Fix the arithmetic used to derive the century byte in
  RTC CMOS, as well as encoding it in BCD.

Reviewed by:	neel
MFC after:	3 days