For install with local imgs, pre-set the image label will not block the install as root will relabel it with root permission.
I'm not sure sec_relable in params is used in unattended_install, do you test with it?
If unattended_install is using virt-install, maybe add the virt-intall option  --security is better for this case.
Another thing is selinux should be enforced for users, and install is the foundation of all tests, so sVirt installation is covered in basic installations somehow.