Fixed upstream:

commit 88f3f7c3901e0b5455796d4213767dd2a4f48045
Author: Peter Krempa <pkrempa@redhat.com>
Date:   Tue Jul 1 16:50:06 2014 +0200

    conf: storage: Add volume feature formatter for gluster pools
    
    Libvirt didn't output feature flags for images stored on native gluster.
    Fix this trivially by adding a feature formatter callback.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1095035

$ git desc
v1.2.6-rc2-15-gf412fc9

Verify this bug with the following package version:
libvirt-1.2.8-11.el7.x86_64
qemu-img-rhev-2.1.2-17.el7.x86_64
kernel-3.10.0-219.el7.x86_64 

Steps:
1. prepare one gluster pool, there is one volume is qcow2v3 format.
# virsh pool-list 
 Name                 State      Autostart 
-------------------------------------------
 default              active     yes       
 gluster-pool         active     no        

# virsh vol-list gluster-pool
 Name                 Path                                    
------------------------------------------------------------------------------
 qcow3                gluster://#glusterServerIP#/gluster-vol1/qcow3
 test.qcow2           gluster://#glusterServerIP#/gluster-vol1/test.qcow2
 test.raw             gluster://#glusterServerIP#/gluster-vol1/test.raw

2. checking the vol-dumpxml of the qcow2v3 format volume, the "<lazy_refcounts/>" is there as expected.
# virsh vol-dumpxml qcow3 gluster-pool
<volume type='network'>
  <name>qcow3</name>
  <key>gluster://#glusterServerIP#/gluster-vol1/qcow3</key>
  <source>
  </source>
  <capacity unit='bytes'>1073741824</capacity>
  <allocation unit='bytes'>197120</allocation>
  <target>
    <path>gluster://#glusterServerIP#/gluster-vol1/qcow3</path>
    <format type='qcow2'/>
    <permissions>
      <mode>0600</mode>
      <owner>0</owner>
      <group>0</group>
    </permissions>
    <timestamps>
      <atime>1419333063.416933449</atime>
      <mtime>1419333063.285934984</mtime>
      <ctime>1419333063.285934984</ctime>
    </timestamps>
    <compat>1.1</compat>
    <features>
      <lazy_refcounts/>
    </features>
  </target>
</volume>


Change the bug status to verify.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0323.html