bhyve: Compile some device models only on amd64

These models register legacy PCI interrupts, which won't be supported in
the arm64 port. In principle it should be possible to make these models
work on arm64 with a bit of effort, so don't move the sources to the
amd64 subdirectory.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40738

(cherry picked from commit 71cc76e8d78eba4f6ff158c1d163af573a52580e)