tools/libxl: Switch Arm guest type to PVH

tools/libxl: Switch Arm guest type to PVH

Currently, the toolstack is considering Arm guest always PV. However,
they are very similar to PVH because HW virtualization extension are used
and QEMU is not started. So switch Arm guest type to PVH.

To keep compatibility with toolstack creating Arm guest with PV type
(e.g libvirt), libxl will now convert those guests to PVH.

Furthermore, the default type for Arm in xl will now be PVH to allow
smooth transition for user.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>