Add debug log for pci passthrough filter

Sometimes operator need information why the host doesn't pass
the check of scheduler, this patch adds information for pci
passthrough filter if it falied to pass check.

Change-Id: Ic8fe13174fda5c75ce95fc0b64391ae081b36c6d
Partial-Bug: #1301830

nova/scheduler/filters/pci_passthrough_filter.py

@@ -13,8 +13,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from nova.openstack.common import log as logging
 from nova.scheduler import filters
 
+LOG = logging.getLogger(__name__)
+
 
 class PciPassthroughFilter(filters.BaseHostFilter):
     """Pci Passthrough Filter based on PCI request
@@ -34,7 +37,12 @@ class PciPassthroughFilter(filters.BaseHostFilter):
 
     def host_passes(self, host_state, filter_properties):
         """Return true if the host has the required PCI devices."""
-        if not filter_properties.get('pci_requests'):
+        pci_requests = filter_properties.get('pci_requests')
+        if not pci_requests:
             return True
-        return host_state.pci_stats.support_requests(
-            filter_properties.get('pci_requests'))
+        if not host_state.pci_stats.support_requests(pci_requests):
+            LOG.debug("%(host_state)s doesn't have the required PCI devices"
+                      " (%(requests)s)",
+                      {'host_state': host_state, 'requests': pci_requests})
+            return False
+        return True