Add opt_pci.h include to fix NO_PCI_MSI_MSIX build.
(Path from Paolo Pisati in current_users@)

While here:
Simplify mp_cpu_start() ifdefs. MULTIPROCESSOR and HYPERV code falls under
NLAPIC > 0, thus just combine all blocks under this guard.
Rearrange opt_acpi.h include alphabetically.

(andvar)