KVM: s390: improve debug feature usage

We do not use the exception logger, so the 2nd area is unused.
Just have one area that is bigger (32 pages).
At the same time we can limit the debug feature size to 7
longs, as the largest user has 3 parameters + string + boiler
plate (vCPU, PSW mask, PSW addr)

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>