virSecuritySELinuxSetFileconHelper: Don't fail on read-only NFS
    
    https://bugzilla.redhat.com/show_bug.cgi?id=996543
    
    When starting up a domain, the SELinux labeling is done depending on
    current configuration. If the labeling fails we check for possible
    causes, as not all labeling failures are fatal. For example, if the
    labeled file is on NFS which lacks SELinux support, the file can still
    be readable to qemu process. These cases are distinguished by the errno
    code: NFS without SELinux support returns EOPNOTSUPP. However, we were
    missing one scenario. In case there's a read-only disk on a read-only
    NFS (and possibly any FS) and the labeling is just optional (not
    explicitly requested in the XML) there's no need to make the labeling
    error fatal. In other words, read-only file on read-only NFS can fail to
    be labeled, but be readable at the same time.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>