O problema mais provável não está relacionado com os direitos de Freems nfs, mas com a configuração do host do selinux (ou apparmor, se você usa o Ubuntu).
De acordo com a documentação da libvirt link
SELinux basic confinement The basic SELinux protection for QEMU virtual machines is intended to protect the host OS from a compromised virtual machine process. There is no protection between guests.
In the basic model, all QEMU virtual machines run under the confined domain root:system_r:qemu_t. It is required that any disk image assigned to a QEMU virtual machine is labelled with system_u:object_r:virt_image_t. In a default deployment, package vendors/distributor will typically ensure that the directory /var/lib/libvirt/images has this label, such that any disk images created in this directory will automatically inherit the correct labelling. If attempting to use disk images in another location, the user/administrator must ensure the directory has be given this requisite label. Likewise physical block devices must be labelled system_u:object_r:virt_image_t.
Not all filesystems allow for labelling of individual files. In particular NFS, VFat and NTFS have no support for labelling. In these cases administrators must use the 'context' option when mounting the filesystem to set the default label to system_u:object_r:virt_image_t. In the case of NFS, there is an alternative option, of enabling the virt_use_nfs SELinux boolean.
Para verificar se os rótulos corretos estão lá, faça um ls -Z no diretório em que você armazena as imagens da VM.
Para ativar o acesso do nfs à vm, você precisa usar:
setsebool virt_use_nfs on