tools/virtio: Add --batch option

This allow to test vhost having >1 buffers in flight

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200401183118.8334-5-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20200418102217.32327-3-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>