hw/pcie: fix the generic pcie root port to support migration

Add msix state to pcie-root-ports's vmstate
in order to support migration.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>