[S390] Replace nopav-message on VM.

Specifying kernel parameter "dasd=nopav" on systems running under VM
has no function but results in message "disable PAV mode". Correct
message is "'nopav' not supported on VM".

Signed-off-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>