restapi: missing disks link in SD VMs resource

Added missing 'disks' link for vms/templates under an export
storage domain resource (i.e. on locations [1] and [2]).

[1] /api/storagedomains/{storagedomain:id}/vms
[2] /api/storagedomains/{storagedomain:id}/templates

Change-Id: Ic8b7fe696f66eb02f2d402985e845f746ccb2095
Bug-Url: https://bugzilla.redhat.com/1130182
Signed-off-by: Daniel Erez <derez@redhat.com>