CA-154341: Disallow Resume when a Checkpoint is ongoing

The Resume operation sould be blocked when a checkpoint is ongoing,
otherwise two domains may coexist for a single VM.  In existent
implement the current-operations field of a VM were cleaned upon
the finish of the suspension, which is a part of a checkpoint task,
makes Resume become an allowed operation.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>