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>