[S390] Use IPL CLEAR for reipl under z/VM

If we use the CLEAR ipl option, reipl is faster, since then VM can release
the memory, which has been paged out.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>