restapi: BackendExportDomainDisksResourceTest build

Fix the build break introduced in change-id
I83fd4b046f72f73bb516615137522ac40028a7f0I83fd4b046f72f73bb516615137522ac40028a7f0
(using setimageStatus instead of setImageStatus).

Change-Id: I7e1a303a00826d3d1f07b648ef97bd5d8c07a21b
Signed-off-by: Allon Mureinik <amureini@redhat.com>