bhyve: Partially disable INT#x support in virtio for arm64

A FreeBSD guest won't make use of this support and pci_lintr_* is not
implemented on arm64. Simply make pci_lintr_*() calls amd64-specific
for now.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41741