KVM: s390: Stop using CONFIG_PGSTE

KVM: s390: Stop using CONFIG_PGSTE

Switch to using IS_ENABLED(CONFIG_KVM) instead of CONFIG_PGSTE, since
the latter will be removed soon.

Many CONFIG_PGSTE are left behind, because they will be removed
completely in upcoming patches. The ones replaced here are mostly the
ones that will stay.

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>