Add support for CDROM devices for bhyve driver using
bhyve(8)'s 'ahci-cd' device type.
As bhyve currently does not support media insertion at runtime,
disallow to start a domain with an empty source path for cdrom
devices.
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
