spec: Restrict virt-login-shell usage
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1033614
    
    As virt-login-shell is an SUID binary, we should restrict its usage to
    just the users chosen by an administrator to use virt-login-shell as
    their login shell. This can easily be done by making the binary
    executable only by users from a new virtlogin group.