KVM: async_pf: Provide additional direct page notification

KVM: async_pf: Provide additional direct page notification

By setting a Kconfig option, the architecture can control when
guest notifications will be presented by the apf backend.
There is the default batch mechanism, working as before, where the vcpu
thread should pull in this information.
Opposite to this, there is now the direct mechanism, that will push the
information to the guest.
This way s390 can use an already existing architecture interface.

Still the vcpu thread should call check_completion to cleanup leftovers.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
(cherry picked from commit e0ead41a6dac09f86675ce07a66e4b253a9b7bd5)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>