libxl: introduce a PVH guest type

libxl: introduce a PVH guest type

The new guest type is introduced to the libxl IDL. libxl__domain_make
is also modified to save the guest type, and libxl__domain_type is
expanded to fetch that information when detecting guest type.

This is required because the hypervisor only differentiates between PV
and HVM guests, so libxl needs some extra information in order to
differentiate between a HVM and a PVH guest.

The new PVH guest type and its options are documented on the xl.cfg
man page.

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