Fix libvirt plugin bug for newly created VMs

Once the instance cache file is built, we can never refresh the
cache because we get a TypeError trying to build the dimensions
and mis-diagnose the VM as a ghost VM.  Simple fix is to move
the update cache call above where we look for the instance name
in the cache.

Change-Id: Iddc229a4e0f150e838a82ace36bc1ba92be17a0e
Closes-Bug: #1486688

monasca_agent/collector/checks_d/libvirt.py

@@ -168,6 +168,8 @@ class LibvirtCheck(AgentCheck):
         for inst in insp._get_connection().listAllDomains():
             # Verify that this instance exists in the cache.  Add if necessary.
             inst_name = inst.name()
+            if inst_name not in instance_cache:
+                instance_cache = self._update_instance_cache()
 
             # Build customer dimensions
             try:
@@ -196,8 +198,6 @@ class LibvirtCheck(AgentCheck):
                 self.gauge('vm.host_alive_status', 2, dimensions=dims_operations,
                            value_meta={'detail': detail})
                 continue
-            if inst_name not in instance_cache:
-                instance_cache = self._update_instance_cache()
             if inst_name not in metric_cache:
                 metric_cache[inst_name] = {}