vfio/iommufd: Relax assert check for iommufd backend

vfio/iommufd: Relax assert check for iommufd backend

Currently iommufd doesn't support dirty page sync yet,
but it will not block us doing live migration if VFIO
migration is force enabled.

So in this case we allow set_dirty_page_tracking to be NULL.
Note we don't need same change for query_dirty_bitmap because
when dirty page sync isn't supported, query_dirty_bitmap will
never be called.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>