tools/xl: Fix segfaults from `xl psr-cat-cbm-set` command line handling

tools/xl: Fix segfaults from `xl psr-cat-cbm-set` command line handling

The socket option takes a mandatory argument.  Mark it as such, so
optarg isn't NULL when passed to trim(), which unconditionally
dereference it.

Range check optind against argc before blindly assuming that
argv[optind] and argv[optind+1] exist.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Chao Peng <chao.p.peng@linux.intel.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>