virtio: integrate into the build and image

* device_manager: scans busses/virtio for network device types
  and scsi controllers.
Virtio: added drivers for PCI busses, bus manager and block device.

* the Virtio PCI bus driver exposes a Virtio controller to the Virtio bus manager,
which in turn exposes a Virtio device consumed by Virtio drivers. Drivers follow the
new driver model.
* virtio_block handles Virtio block devices under disk/virtual/virtio_block/x/raw.
* Here is the Qemu command line option for Virtio disk devices:
  -drive file=haiku.image,if=virtio
* the PCI bus driver currently supports only legacy interrupts (no MSI(-X) yet).
* There is room for improvements in the bus manager:
  - it notifies the host for each queued request, which isn't optimal.
  - transfer descriptors should probably be simply preallocated (they are nicely
    leaked at the moment).
  - indirect descriptors are not supported yet.
and in the block driver:
  - get the id of the disk.
  - implements flushing the cache.
  - improves dma restrictions.
  - do_io() should use a page for header descriptors instead of malloc(), which
    could cross boundaries.
* The device manager tries to guess the driver based on the PCI device type, this
implies having to declare the "busses/virtio" path for each possible type
provided by Virtio. Thus future driver additions might require patching the device
manager.
* virtio.h is still private, the API is subject to changes.
* virtio_pci.h, virtio_blk.h, virtio_ring.h are copied unchanged from FreeBSD.
Merge branch 'virtio'