xl: Command line: Adjust "Fix segfaults from `xl psr-cat-cbm-set`..."

xl: Command line: Adjust "Fix segfaults from `xl psr-cat-cbm-set`..."

This adjust commit a49077e5 "Fix segfaults from `xl psr-cat-cbm-set`
command line handling":

 * Do not use the constant `required_argument' here (we simply use 1
   everywhere else).

 * Fix the minimum required arguments argument to SWITCH_FOREACH_OPT.

Leave the separate check on optind, because it checks for too many as
well as too few arguments.

(There are many things in xl which fail to check for too many
arguments.  I do not intend to drain that swamp now: I started but
decided a complete overhaul of most of xl's command line argument
processing would be best.)

This is just a code cleanup with no ultimate functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Chao Peng <chao.p.peng@linux.intel.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>