Handle missing libvirt vnic targets!

Updates the libvirt inspector so that our vnic inspection
can handle cases where the interface/target is missing.
This can occur in some (usually transient) cases where
Nova is pausing/migrating/etc an instance.

Previously Ceilometer would functionally work fine but
a rather long stack trace would get logged to the log file.

Fixes LP Bug #1221784

Change-Id: I7113c6e4bb4af622d517915d13147f5912bad126

ceilometer/compute/virt/libvirt/inspector.py

@@ -111,7 +111,11 @@ class LibvirtInspector(virt_inspector.Inspector):
         domain = self._lookup_by_name(instance_name)
         tree = etree.fromstring(domain.XMLDesc(0))
         for iface in tree.findall('devices/interface'):
-            name = iface.find('target').get('dev')
+            target = iface.find('target')
+            if target is not None:
+                name = target.get('dev')
+            else:
+                continue
             mac = iface.find('mac').get('address')
             fref = iface.find('filterref')
             if fref is not None:

tests/compute/virt/libvirt/test_inspector.py

@@ -47,6 +47,14 @@ class TestLibvirtInspection(test_base.TestCase):
         dom_xml = """
              <domain type='kvm'>
                  <devices>
+                    <!-- NOTE(dprince): interface with no target -->
+                    <interface type='bridge'>
+                       <mac address='fa:16:3e:93:31:5a'/>
+                       <source bridge='br100'/>
+                       <model type='virtio'/>
+                       <address type='pci' domain='0x0000' bus='0x00' \
+                       slot='0x03' function='0x0'/>
+                    </interface>
                     <interface type='bridge'>
                        <mac address='fa:16:3e:71:ec:6d'/>
                        <source bridge='br100'/>