cli: more cleanly separat the notion of 'format' from 'output'

The formats are
 * raw
 * vhd

The outputs are
 * human-readable summary on stdout
 * nbd:// URI

We still need to add the outputs:
 * HTTP(S) PUT
 * XenServer chunked encoding
 * XenServer HTTP-tunnelled NBD

Signed-off-by: David Scott <dave.scott@eu.citrix.com>