libxl: add PVH support to domain creation

libxl: add PVH support to domain creation

Remove the device model "none" support from domain creation and
introduce support for PVH.

This requires changing some of the HVM checks to be applied for both
HVM and PVH.

Setting device model to none was never supported since it was an
unstable interface used while transitioning from PVHv1 to PVHv2.

Now that PVHv1 has been finally removed and that a supported
interface for PVHv2 is being added this option is no longer necessary,
hence it's removed.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>