tools/virtio: Extract virtqueue initialization in vq_reset

So we can reset after that in the main loop.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Link: https://lore.kernel.org/r/20200418102217.32327-7-eperezma@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>