Virtio 0.9 implementation was limited to the maximum virtqueue size of
MAX_QUEUE_NUM and the virtio-net driver would fail to initialize on hosts
exceeding this limit.

This commit lifts the restriction by allocating the queue memory based on
the actual queue size instead of using a fixed maximum. Note that virtio
1.0 still uses the MAX_QUEUE_NUM constant to cap the size (unfortunately
this functionality is not available in virtio 0.9).

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>