vfio/migration: Emit VFIO migration QAPI event

Emit VFIO migration QAPI event when a VFIO device changes its migration
state. This can be used by management applications to get updates on the
current state of the VFIO device for their own purposes.

A new per VFIO device capability, "migration-events", is added so events
can be enabled only for the required devices. It is disabled by default.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
3 files changed