RHEV: Data Center Upgrade to 3.6 fails with "Upgrading a pool while an upgrade is in process is unsupported"

Issue

• Cannot upgrade Data Center compatibility version from 3.5 to 3.6.
• Changing Data Center compatibility version to 3.6 is failing error.
• Getting error while changing Data Center to 3.6 from 3.5.
• In RHEV-M GUI, the following error is displayed:

VDSM command failed: Upgrading a pool while an upgrade is in process is unsupported (pool: `poolID`): ''

• In the manager machine, /var/log/ovirt-engine/engine.log presents the following error:

[org.ovirt.engine.core.vdsbroker.irsbroker.UpgradeStoragePoolVDSCommand] (thread) [ID] Upgrading storage pool 'poolID' to version '3'.
[org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (thread) [ID] Correlation ID: null, Call Stack: null, Custom Event ID: -1, Message: VDSM command failed: Upgrading a pool while an upgrade is in process is unsupported (pool: `poolID`): 
Upgrading a pool while an upgrade is in process is unsupported (pool: `poolID`): '' (Failed with error PoolUpgradeInProgress and code 396)

• In SPM, the following can be seen in /var/log/vdsm/vdsm.log:

jsonrpc.Executor/0::ERROR::date time::task::id::Storage.TaskManager.Task::(_setError) Task=`taskID`::Unexpected error
Traceback (most recent call last):
  File "/usr/share/vdsm/storage/task.py", line 873, in _run
    return fn(*args, **kargs)
  File "/usr/share/vdsm/logUtils.py", line 49, in wrapper
    res = f(*args, **kwargs)
  File "/usr/share/vdsm/storage/hsm.py", line 3539, in upgradeStoragePool
    pool._upgradePool(targetDomVersion)
  File "/usr/share/vdsm/storage/securable.py", line 77, in wrapper
    return method(self, *args, **kwargs)
  File "/usr/share/vdsm/storage/sp.py", line 442, in _upgradePool
    raise se.PoolUpgradeInProgress(self.spUUID)
PoolUpgradeInProgress: Upgrading a pool while an upgrade is in process is unsupported (pool: `poolID`): ''