libxc: panic when trying to create a PVH guest without kernel support

libxc: panic when trying to create a PVH guest without kernel support

Previously when trying to boot a PV capable but not PVH capable kernel
inside of a PVH container xc_dom_guest_type would succeed and return a
PV guest type, which would lead to failures later on in the build
process.

Instead provide a clear error message when trying to create a PVH
guest using a kernel that doesn't support PVH.

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