s390x/async_pf: Check for apf extension and enable pfault

S390 can also use async page faults, to enhance guest scheduling.
In case of live migration we want to disable the feature and let
all pending request finish.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
