xl: Command line: Support -h everywhere

xl: Command line: Support -h everywhere

xl subcommands ought all to take -h.  def_getopt and hence
SWITCH_FOREACH_OPT already handles 'h' by calling helpstr.  None of
the call sites see the 'h'.

In this patch:

 * Change SWITCH_FOREACH_OPT to always add a "h" to the short opts
   string, using string concatenation.

 * Remove the now-redundant h's from some existing option strings.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>