bhyve: Do not compile PCI passthrough support on arm64

[markj]

Summary

Some required kernel functionality is not yet implemented.

For now this means that one cannot specify host PCI register values, but
that functionality is only used by amd64-specific device models for now.
Note that this limitation is rather artificial; it arises only because
pci_host_read_config() lives in pci_passthru.c.

[andrew]

There is some trickiness with PCI passthrough on arm due to memory types that is fixed in later revisions of the architecture.