Fixed upstream:

commit cea1b86fa7f140a30220bb2efaece1d2890f2d91
Author: Erik Skultety <eskultet>
Date:   Fri Apr 17 13:14:12 2015 +0200

    rng: Forbid to validate mismatched <disk> 'device' and 'type' attributes
    
    According to docs, using 'lun' as a value for device attribute is only valid
    with disk types 'block' and 'network'. However current RNG schema also allows
    a combination type='file' device='lun' which results in a successfull
    xml validation, but fails at qemuBuildCommandLine.
    Besides fixing the RNG schema, this patch also adds a qemuxml2argvtest
    for this case.

v1.2.14-223-gcea1b86

Reproduce it on libvirt-1.2.14-1.el7.x86_64

Verified version:
libvirt-1.2.15-2.el7.x86_64

steps:
1.prepare a xml with "disk type='file' and device='lun'"
# cat r7.2.xml | grep disk -A 9
......
<disk type='file' device='lun'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/raw.img'/>
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='4'/>
    </disk>
......
2.validate it with virt-xml-validate , it will report error.
# virt-xml-validate r7.2.xml
Relax-NG validity error : Extra element devices in interleave
r7.2.xml:31: element devices: Relax-NG validity error : Element domain failed to validate content
r7.2.xml fails to validate

3.change domain xml to "disk type='block' " ,it is valid.
# cat r7.2.xml | grep disk -A 9
 <disk type='block' device='lun'>
      <driver name='qemu' type='raw'/>
      <source dev='/dev/sde'/>
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='4'/>
    </disk>

# virt-xml-validate r7.2.xml
r7.2.xml validates

4.change domain xml to "disk type='network' and source protocol is iscsi" ,it is valid.
<disk type='network' device='lun'>
      <driver name='qemu' type='raw'/>
      <source protocol='iscsi' name='iqn.2008-09.5.165.3ffe.104:server.target1/1'>
        <host name='$IP' port='3260'/>
      </source>
      <target dev='sde' bus='scsi'/>
    </disk>
</devices>


Now ,  virt-xml-validate can report error when guest xml configured with "disk type='file' device='lun'" . and it is valid with "block lun " and "iscsi network lun " . 


Note :

Since device "lun" is only valid when type is "block" or "network" using the iSCSI protocol . If the network protocol is not ISCSI , should virt-xml-validate also report error .

5.prepare a xml like following 
# cat test.xml |grep disk -A 9
......
 <disk type='network' device='lun'>  <=== device is lun 
      <driver name='qemu' type='qcow2' cache='none'/>
      <source protocol='gluster' name='gluster-vol1/r6q2.img'>  <===protocol is gluster not iscsi
        <host name='$IP'/>
      </source>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>
......

6.using virt-xml-validate to validate it , it is valid.
# virt-xml-validate test.xml 
test.xml validates

7.using above domain xml to create a guest , it will report error .
# virsh create validate.xml 
error: Failed to create domain from validate.xml
error: unsupported configuration: disk device='lun' is not supported for protocol='gluster'

In step6 , should it also report error if the network protocol is not iscsi . Thanks a lot .

I get your point, but I don't think it's worth it, we can't know for sure that there won't be any support for gluster at all, thus I'd leave this as a post-parse logic rather than fragment our RNG schema where maintainability matters.

Thanks for your info , and according to comment 3 and comment 4 , moving it to verified .

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/RHBA-2015-2202.html