security_dac: honor relabel='no' in disk config

security_dac: honor relabel='no' in disk config

https://bugzilla.redhat.com/show_bug.cgi?id=999301

The DAC driver ignores the relabel='no' attribute in disk config

  <disk type='file' device='floppy'>
    <driver name='qemu' type='raw'/>
    <source file='/some/path/floppy.img'>
      <seclabel model='dac' relabel='no'/>
    </source>
    <target dev='fda' bus='fdc'/>
    <readonly/>
  </disk>

This patch avoid labeling disks when relabel='no' is specified.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Jim Fehlig <jfehlig@suse.com>