Implement Multiple Message MSI support on amd64. This is experimental code

Implement Multiple Message MSI support on amd64.  This is experimental code
to assist qwx(4) development.  We may remove this code again at some point
in the future.

Multiple Message MSI has some serious design flaws, especially when
combined with the APIC interrupt controller architecture.  It was
superseded by MSI-X.  Unfortunately qwx(4) does not implement MSI-X.

ok stsp@, deraadt@