Add status changing and feature negotiation.
Add basic virtqueue support for adding and sending virtqueue requests.
Add ISR checking.

[Squashed request endianness fix by Greg Kurz <gkurz@linux.vnet.ibm.com>
--Stefan]
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMarc Marí <marc.mari.barcelo@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

Add functions to read and write virtio header fields.
Add status bit setting in virtio-blk-device.
Signed-off-by: NMarc Marí <marc.mari.barcelo@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

Virtio header has been changed to compile and work with a real device.
Functions bus_foreach and device_find have been implemented for PCI.
Virtio-blk test case now opens a fake device.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMarc Marí <marc.mari.barcelo@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>