core,ui: VirtIO-SCSI enabled flag

core:
* VmManagementParametersBase -> virtioScsiEnabled Boolean.
** Using a nullable boolean for setting a default value
   when not specified (null value).
* When enabled, a VirtIO-SCSI controller device is added to vm_device table.
** Used for maintaining stable address to the pci controller.
** Necessary for hot-plugging a disk with VirtIO-SCSI interface.
* The flag is defaulted to true on cluster >= 3.3.

UI:
* Added 'VirtIO-SCSI Enabled' check-box in VM dialog
  (under 'Resource Allocation' tab -> 'Storage Allocation')
* Screenshot: http://imgur.com/zP7lxsv

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

Change-Id: Ice06fda5946b5961cfe9c819b0b368525b1f4b58
Signed-off-by: Daniel Erez <derez@redhat.com>