staging: unisys: visorbus: bus_responder add error handling

Controlvm_respond now returns an error. Add error handling to the
bus_responder function.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>