Make it possible to query all net clients without specifying an ID when calling
qemu_find_net_clients_except().

This also adds the add_completion_option() function which is to be used for
other commands completions as well.
Signed-off-by: NHani Benhabiles <hani@linux.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>