Make bhyveload respect boot order as specified by os.boot section of the
domain XML or by "boot order" for specific devices. As bhyve does not
support a real boot order specification right now, it's just about
choosing a single device to boot from.

Automatically allocate PCI addresses for devices instead
of hardcoding them in the driver code. The current
allocation schema is to dedicate an entire slot for each devices.

Also, allow having arbitrary number of devices.

At this point unittest covers 4 basic cases:

 - minimal working XML for bhyve
 - same as above, but with virtio disk
 - ACPI and APIC args test
 - MAC address test