bhyve: Support legacy PCI interrupts on arm64

This allows us to remove various #ifdef hacks and enable building more
PCI devices.

Note that a hole is left in the interrupt mapping for the RTC rather
than having the two core devices straddle the PCIe interrupts. QEMU's
virt machine also takes this approach.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD