s390/dasd: check for availability of prefix command during format

The prefix command is used instead of a define extent to make use of
PAV alias devices during format. On some older storage servers the
prefix command may not be available and the IO request will fail.
Check for availability of prefix command and use define extent if
not available.

Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>