libxl: switch HVM domain building to use xc_dom_* helpers

libxl: switch HVM domain building to use xc_dom_* helpers

Now that we have all the code in place HVM domain building in libxl can be
switched to use the xc_dom_* family of functions, just like they are used in
order to build PV guests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>