tools/virtio: Add --reset

Currently, it only removes and add backend, but it will reset vq
position in future commits.

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