Check for null just in case

This code was recently switched from using HashMap to ConcurrentHashMap.
ConcurrentHashMap does not accept null keys and will through a NPE.  Adding
a null check just in case somebody passes null.

server/src/com/cloud/hypervisor/HypervisorGuruManagerImpl.java

@@ -57,6 +57,10 @@ public class HypervisorGuruManagerImpl extends ManagerBase implements Hypervisor
 
     @Override
     public HypervisorGuru getGuru(HypervisorType hypervisorType) {
+        if (hypervisorType == null) {
+            return null;
+        }
+
         HypervisorGuru result = _hvGurus.get(hypervisorType);
 
         if ( result == null ) {