bhyve: Move the gvt-d driver to amd64/

It is amd64-only. No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40736

(cherry picked from commit 4ab7aea8ec2f5fac0e6d140300c8f04db99d83df)