Expose a new argument for tunneled migration of a VM

Tunneled migration (notice the US spelling!) provides better
authentication and encryption for migration data, in the price of higher
CPU consumption and libvirtd latency. Let Engine choose if this is
wanted per migration command.

Change-Id: Id713b455db4f9bf437720b64c92bba9377cd89ae
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/2551
Reviewed-by: Moti Asayag <masayag@redhat.com>
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Tested-by: Omer Frenkel <ofrenkel@redhat.com>