CLOUDSTACK-5401: VM migration during host maintenance fails if pool.storage.capacity.disablethreshold is lowered

Changes:
- During Vm migration while finding a new host within the cluster, we need to set the storagepool Id to the deployment plan too.
- This will indicate the planner that the volumes are ready and no need to find new pool
- This in turn will prevent the threshold check done during the pool allocation. This step is not needed since there is no need to allocate pools newly.
- Thus the migration wont fail because th threshold check fails.

engine/orchestration/src/com/cloud/vm/VirtualMachineManagerImpl.java

@@ -2054,8 +2054,16 @@ public class VirtualMachineManagerImpl extends ManagerBase implements VirtualMac
         }
 
         Host host = _hostDao.findById(hostId);
+        Long poolId = null;
+        List<VolumeVO> vols = _volsDao.findReadyRootVolumesByInstance(vm.getId());
+        for (VolumeVO rootVolumeOfVm : vols) {
+            StoragePoolVO rootDiskPool = _storagePoolDao.findById(rootVolumeOfVm.getPoolId());
+            if (rootDiskPool != null) {
+                poolId = rootDiskPool.getId();
+            }
+        }
 
-        DataCenterDeployment plan = new DataCenterDeployment(host.getDataCenterId(), host.getPodId(), host.getClusterId(), null, null, null);
+        DataCenterDeployment plan = new DataCenterDeployment(host.getDataCenterId(), host.getPodId(), host.getClusterId(), null, poolId, null);
         ExcludeList excludes = new ExcludeList();
         excludes.addHost(hostId);