spapr/xive: introduce a VM state change handler

spapr/xive: introduce a VM state change handler

This handler is in charge of stabilizing the flow of event notifications
in the XIVE controller before migrating a guest. This is a requirement
before transferring the guest EQ pages to a destination.

When the VM is stopped, the handler sets the source PQs to PENDING to
stop the flow of events and to possibly catch a triggered interrupt
occuring while the VM is stopped. Their previous state is saved. The
XIVE controller is then synced through KVM to flush any in-flight
event notification and to stabilize the EQs. At this stage, the EQ
pages are marked dirty to make sure the EQ pages are transferred if a
migration sequence is in progress.

The previous configuration of the sources is restored when the VM
resumes, after a migration or a stop. If an interrupt was queued while
the VM was stopped, the handler simply generates the missing trigger.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190513084245.25755-6-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>