bhyve: Move more amd64-specific code under amd64/

mptable and the e820 are both rather amd64-specific and can be moved
easily.

In the case of e820, move the registration with qemu_fwcfg into e820.c,
as it simplifies bhyverun.c a bit and I can't see any downsides.

No functional change intended.

Reviewed by: corvink, jhb, emaste
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40552