Allow commands having a NULL cmd pointer, add a function to set the
pointer later.  Use case: allow modules implement hmp commands.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jose R. Ziviani <jziviani@suse.de>
Message-Id: <20210624103836.2382472-31-kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>