Skip any files which are not mounted on the host
    
    Currently the LXC container tries to skip selinux/securityfs
    mounts if the directory does not exist in the filesystem,
    or if SELinux is disabled.
    
    The former check is flawed because the /sys/fs/selinux
    or /sys/kernel/securityfs directories may exist in sysfs
    even if the mount type is disabled. Instead of just doing
    an access() check, use an virFileIsMounted() to see if
    the FS is actually present in the host OS. This also
    avoids the need to check is_selinux_enabled().
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>