Add debug log for core_filter

Sometimes operator need information why the host doesn't pass
the check of scheduler, this patch adds information for core
filter if virtual cpu is not enough on the host.

Change-Id: Ie05d9184435a4fc4cff1af26812c01a3cc226fff
Partial-Bug: #1301830

nova/scheduler/filters/core_filter.py

@@ -61,7 +61,17 @@ class BaseCoreFilter(filters.BaseHostFilter):
         if vcpus_total > 0:
             host_state.limits['vcpu'] = vcpus_total
 
-        return (vcpus_total - host_state.vcpus_used) >= instance_vcpus
+        free_vcpus = vcpus_total - host_state.vcpus_used
+        if free_vcpus < instance_vcpus:
+            LOG.debug("%(host_state)s does not have %(instance_vcpus)d "
+                      "usable vcpus, it only has %(free_vcpus)d usable "
+                      "vcpus",
+                      {'host_state': host_state,
+                       'instance_vcpus': instance_vcpus,
+                       'free_vcpus': free_vcpus})
+            return False
+
+        return True
 
 
 class CoreFilter(BaseCoreFilter):