libvmmapi: Add a subdirectory for amd64-specific code

Move vmmapi_freebsd.c there. It contains x86-specific code used only by
bhyveload(8).

Move vcpu_reset() into vmmapi_machdep.c. It is also x86-specific.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40998