tools: Prepare further command line tool extensions

tools: Prepare further command line tool extensions

Allow to add more extension scripts to the command line tool. We define
a structure that describes an extension by command / subcommand and
provides help to be displayed by the tool. The extension script itself
has to be called jailhouse-<command>-<subcommand>. We look for it in
$PATH, extended by the tool's directory and /usr/lib/jailhouse.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>