rest: VirtIO-SCSI enabled flag

* Added 'virtio_scsi_enabled' boolean to VM/Template resource (api.xsd).
* Set 'VirtioScsiEnabled' value in:
  BackendVmsResource (add VM) / BackendVmResource (update VM)
  BackendTemplatesResource (add template) / BackendTemplateResource (update tempalte)
* Added VmHelper -> getVirtioScsiControllersForEntity
* Updated rsdl
* Updated tests
* Usage:
  an indication on new/edit VM for whether to attach a VirtIO-SCSI
  controller upon VM start (defaulted to true on cluster >= 3.3).

Feature page: http://www.ovirt.org/Features/Virtio-SCSI

Change-Id: I481025152b20579ac615d9023ed0cef9b64558e1
Bug-Url: https://bugzilla.redhat.com/1014491
Signed-off-by: Daniel Erez <derez@redhat.com>