staging: unisys: visorbus: Update vmcallinterface comment

The comment at the beginning of the vmcallinterface.h file references IO
Virtualization. It should be more specific that it is referencing s-Par
Virtualization.

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>