Pull up following revision(s) (requested by tsutsui in ticket #819):
sys/arch/amd64/amd64/mainbus.c: revision 1.28 via patch
sys/arch/i386/i386/mainbus.c: revision 1.85 via patch
sys/arch/x86/x86/mpacpi.c: patch
sys/arch/x86/x86/mpbios.c: patch
Apply fixes from jmcneill@ for PR port-i386/38729
(ACPI kernel booted under qemu cannot detect devices):
- make MP SCANPCI function for ACPI_SCANPCI and MPBIOS_SCANPCI
  return a number of attached PCI busses
- if no valid PCI busses are attached in the MP SCANPCI function,
  try to probe and attach pci0 at mainbus as well as kernels
  with no SCANPCI options

(snj)