restapi : API response sent not in XML format

When sending a request to unplug a disk from vm via REST,
the response received from API was not in XML format.

Change-Id: I9e1a9e8c5283efcd4dba31b0f1fa0414adc266fb
Bug-Url: https://bugzilla.redhat.com/981989
Signed-off-by: Ravi Nori <rnori@redhat.com>