vringh_getdesc_iotlb() manages 2 iovs for writable and readable
descriptors. This is very useful for the block device, where for
each request we have both types of descriptor.

Let's split the vdpasim_virtqueue's iov field in out_iov and
in_iov to use them with vringh_getdesc_iotlb().

We are using VIRTIO terminology for "out" (readable by the device)
and "in" (writable by the device) descriptors.
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-18-sgarzare@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>