CLIHandler: use resolved command definition

For sub commands we resolve the real $cmd, $def and its arguments,
thus we should also get the handler from the resolved $def, not the
global one.

No change for normal (consisting of only the first argument)
commands, for them $cmddef == $def.
This sneaked in in a respin/rebase of the series.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>