[libvirt] images on NFS do not have MCS when selinux is on

hi,all i now test selinux(enforcing). i assign a nfs-image to a VM, then start it. at the beginning i set virt_use_nfs on, its image does not has a MCS corresponding to qemu-kvm process's MCS then i remount nfs with -o context="system_u:object_r:virt_image_t:s0" , restart VM, its image does not has a corresponding MCS. #ls -lZ ####system_u:object_r:virt_image_t:s0 803003d2-3a2b-4581-a7cb-dc7fac06e7af why this happen? if this is right for NFS? thanks