virtio: free fDescriptors on VirtioQueue destruction.
Virtio: added a driver with basic support for SCSI devices.

* Here is the Qemu command line option for Virtio SCSI devices:
-drive if=none,id=hd,file=haiku.image -device virtio-scsi-pci,id=scsi -device scsi-hd,drive=hd
* virtio_scsi.h is copied unchanged from FreeBSD, except for the _PACKED directive.
virtio_block: replaced __packed with _PACKED
virtio_scsi: added to the image.

* device_manager: scans busses/scsi for generic drivers.