block/archipelago: Add support for creating images

qemu-img archipelago:<volumename>[/mport=<mapperd_port>[:vport=<vlmcd_port>]
 [:segment=<segment_name>]] [size]

Signed-off-by: Chrysostomos Nanakos <cnanakos@grnet.gr>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>