vfio: unplug failover primary device before migration

vfio: unplug failover primary device before migration

As usual block all vfio-pci devices from being migrated, but make an
exception for failover primary devices. This is achieved by setting
unmigratable to 0 but also add a migration blocker for all vfio-pci
devices except failover primary devices. These will be unplugged before
migration happens by the migration handler of the corresponding
virtio-net standby device.

Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <20191029114905.6856-12-jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>