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
(cherry picked from commit 4fe5b70cae6761b9205ff35b72ccfe60d7326301)