KVM: s390: Allow userspace to limit guest memory size

With commit c6c956b8 ("KVM: s390/mm: support gmap page tables with less
than 5 levels") we are able to define a limit for the guest memory size.

As we round up the guest size in respect to the levels of page tables
we get to guest limits of: 2048 MB, 4096 GB, 8192 TB and 16384 PB.
We currently limit the guest size to 16 TB, which means we end up
creating a page table structure supporting guest sizes up to 8192 TB.

This patch introduces an interface that allows userspace to tune
this limit. This may bring performance improvements for small guests.

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