xl: fixup command line handling for several commands.

xl: fixup command line handling for several commands.

def_getopt already checks for a minimum number of arguments for us.

"xl save" simply need to use the correct argument for that value,
contrary to the change I made in 23876:b113d626cfaf

"xl block-list" does not need to check for at least 2 arguments, since
it's already been done by def_getopt.

"xl network-list" would previous accept zero arguments and just print
the table header. Insist on a domain argument.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>