s390x/kvm: enable/reset cmma via vm attributes

Exploit the new api for userspace-controlled cmma. If supported, enable
cmma during kvm initialization and register a reset handler for cmma,
which is also called directly from the load IPL code.

The reset functionality is needed to reset the cmma state of the guest
pages, e.g. if a system reset is triggered via qemu monitor; otherwise
this could result in data corruption.

A guest triggered reboot may now lead to multiple cmma resets; this is
OK, however, as this is slowpath anyway and the simplest way to achieve
the intended effects.

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