bhyve/pci_emul.c: Emit ACPI methods for hotpluggable slots

[bnovkov]

Summary

This change adds ACPI methods and metadata for PCI hotplugging.
Under this scheme, each virtual PCI bus now emits three DWORD-sized variables,
a mutex, a method for notifying the guest about hotplug events on the
bus (PCNT), and a method signalling a successful device detach to the
hypervisor (PCEJ). The three variables (PCUP, PCDW, and EACK)
are used to signal pending hotadd, hotremove, and completed
eject requests respectively. All hotplug events are injected into
the guest via a GPE interrupt