Description of problem:
The sub-element for panic device should be "interleave" in rng file

Version-Release number of selected component:
libvirt-3.2.0-6.virtcov.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Check the definition of panic device in domaincommon.rng:
# cat /usr/share/libvirt/schemas/domaincommon.rng
......
  <define name="panic">
    <element name="panic">
      <optional>
        <attribute name="model">
          <choice>
            <value>isa</value>
            <value>pseries</value>
            <value>hyperv</value>
            <value>s390</value>
          </choice>
        </attribute>
      </optional>
      <optional>
        <ref name="alias"/>
      </optional>
      <optional>
        <ref name="address"/>
      </optional>
    </element>
  </define>
......

2.
1) Prepare a guest xml with panic device like below:
# cat vm1.xml
......
    <panic model='isa'>
      <alias name='panic0'/>
      <address type='isa' iobase='0x505'/>
    </panic>
......

2) Validate it:
# virt-xml-validate /tmp/vm1.xml 
/tmp/vm1.xml validates

3.
1) Switch the order of alias and address:
# cat vm1.xml
......
    <panic model='isa'>
      <address type='isa' iobase='0x505'/>
      <alias name='panic0'/>
    </panic>
......

2) Validate again:
# virt-xml-validate /tmp/vm1.xml 
Relax-NG validity error : Extra element devices in interleave
/tmp/vm1.xml:39: element devices: Relax-NG validity error : Element domain failed to validate content
/tmp/vm1.xml fails to validate


Actual results:
In step3, xml validation failed.

Expected results:
In step3, xml validation should succeed. This requires that the sub-element for panic device be "interleave" in rng, like:
  <define name="panic">
    <element name="panic">
      <optional>
        <attribute name="model">
          <choice>
            <value>isa</value>
            <value>pseries</value>
            <value>hyperv</value>
            <value>s390</value>
          </choice>
        </attribute>
      </optional>
    **<interleave>**
        <optional>
          <ref name="alias"/>
        </optional>
        <optional>
          <ref name="address"/>
        </optional>
    **</interleave>**
    </element>
  </define>

Additional info:
And I can't find alias info for panic device in active dumpxml after start guest successfully.