virtio: Allow guest to defer VIRTIO_F_NOTIFY_ON_EMPTY (Alex Williamson)

There may be cases where the guest does not want the avail queue
interrupt, even when it's empty.  For the virtio-net case, the
guest may use a different buffering scheme or decide polling for
used buffers is more efficient.  This can be accomplished by simply
checking for whether the guest has acknowledged the existing notify
on empty flag.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6865 c046a42c-6fe2-441c-8c8c-71466251a162
