vmm.4: Update to mention non-amd64 platforms

Most of the text here relates to PCI passthrough, which is still
amd64-only, but we should still document supported platforms.

While here, remove the comment that vmm.ko has to be loaded at boot-time
for passthrough, as devctl makes it possible to detach host drivers on
the fly. I'm not aware of any other reason to require vmm.ko to be
loaded at boot.

Reviewed by: br, andrew
Differential Revision: https://reviews.freebsd.org/D48263