When using qemuProcessAttach to attach a qemu process,
the DAC label is not filled correctly.

Introduce a new function to get the uid:gid from the system
and fill the label.

This fixes the daemon crash when 'virsh screenshot' is called:
https://bugzilla.redhat.com/show_bug.cgi?id=1161831

It also fixes qemu-attach after the prerequisite of this patch
(commit f8c1fb3d) was pushed out of order.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>