dashboard: add basic libvirt domain validator

circle/dashboard/virtvalidator.py

+from django.core.exceptions import ValidationError
+from lxml import etree as ET
+
+
+rng_file = "/usr/share/libvirt/schemas/domain.rng"
+
+header = """<domain type='kvm'>
+  <name>validator</name>
+  <memory unit='KiB'>1024</memory>
+  <os>
+    <type>hvm</type>
+  </os>"""
+footer = """</domain>"""
+
+
+def domain_validator(value):
+    xml = header + value + footer
+    relaxng = ET.RelaxNG(file=rng_file)
+    if not relaxng.validate(ET.fromstring(xml)):
+        raise ValidationError("%s is not valid libvirt Domain xml." % value)