intel_iommu: allow migration

IOMMU needs to be migrated before all the PCI devices (in case there are
devices that will request for address translation). So marking it with a
priority higher than the default (which PCI devices and other belong).
Migration framework handled the rest.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>