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.