staging: unisys: visorbus: add error handling to handle_command

Add error handling to the function handle_command.

Since handle_command now returns errors, we need to process them correctly
in process_controlvm_event.

Note: It is invalid to have a controlvm test message in the event queue, so
return -EINVAL if one is discovered.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>