Allow PCI devices to be configured on all valid bus numbers from 0 to 255.

This is done by representing each bus as root PCI device in ACPI. The device
implements the _BBN method to return the PCI bus number to the guest OS.

Each PCI bus keeps track of the resources that is decodes for devices
configured on the bus: i/o, mmio (32-bit) and mmio (64-bit). These windows
are advertised to the guest via the _CRS object of the root device.

Bus 0 is treated specially since it consumes the I/O ports to access the
PCI config space [0xcf8-0xcff]. It also decodes the legacy I/O ports that
are consumed by devices on the LPC bus. For this reason the LPC bridge can
be configured only on bus 0.

The bus number can be specified using the following command line option
to bhyve(8): "-s <bus>:<slot>:<func>,<emul>[,<config>]"

Discussed with:	grehan@
Reviewed by:	jhb@