restapi : vm/template Import candidates should have /disks (#829672)

When importing vm/template via api only import candidate metadata is
visible (disks are not).

This patch exposes the disks subcollection of vm/template in
the export domain.

Change-Id: I83fd4b046f72f73bb516615137522ac40028a7f0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=829672
Signed-off-by: Ravi Nori <rnori@redhat.com>