Fixes 6621 - Update host memory stats

Each time getMemStat() is called, a static value is returned. This value
should instead be refreshed to return the actual memory used.

Author: Rubueno

plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/LibvirtComputingResource.java

@@ -536,6 +536,7 @@ public CPUStat getCPUStat() {
     }
 
     public MemStat getMemStat() {
+        _memstat.refresh();
         return _memStat;
     }