xl: refactor common parts of command line parsing

xl: refactor common parts of command line parsing

xl command options are currently handled in each command's sub function,
leading to a lot of duplicate code.

This patch moves the common part of it into a separate function,
which handles the help switch, unknown options and an insufficient
number of parameters. This removes a lot of redundant code.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>