various fixes and improvements for AMD-Vi

[Andriy Gapon]

I have created a fake GitHub pull request to try to get some review for a few
AMD-specific changes to vmm.

I would like to ask for a wider review of two of those changes.
They require a bit of knowledge of the acpi and pci bus code (msi setup, etc):

https://github.com/avg-I/freebsd/pull/1/commits/b4ce26a221fe2ab1e3ab1d373575131545b76aa6
https://github.com/avg-I/freebsd/pull/1/commits/eecf82121fc4fedc577af4b3c3b4de455ae6dad8

Thank you!

-- 
Andriy Gapon