Untangle configuration ifdefs a little. On x86, msi is optional on pci,
and also on apic in common and i386 files (except for xen it is optional
only on xenhvm), but it was not ifdefed except on apic in common and i386
files.

This is all that is left from an attempt to build a (sub-)minimal kernel
without any devices. The isa "option" is still used without ifdefs in many
standard files even on amd64. ISAPNP is not optional on at least i386.
ATPIC is not optional on i386 (it is used mainly for Xspuriousint). But
pci is now supposed to be optional on x86.