xen: Ensure host Python2 is used

To avoid Xen build issues ensure that host Python 2 is used.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>