kvm tools: implement "help xxx" command

'kvm run --help' works fine but 'kvm help run' shows nothing,
this patch implements it.

Acked-by: Prasad Joshi <prasadjoshi124@gmail.com>
Signed-off-by: WANG Cong <amwang@redhat.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>