Make xsegbd query peer for disk size(using X_INFO)

Make xsegbd query peer for disk size(using X_INFO)

xsegbd will now send an X_INFO request to learn the size of the disk it's
mapping, instead of using the sector_size parameter at module load time.

Currently, xsegbd allocates its own segment, and thus it must wait until blockd
(or another peer with X_INFO support) joins the segment to get a response (this
will probably change soon).