libxc: introduce a xc_dom_arch for hvm-3.0-x86_32 guests

libxc: introduce a xc_dom_arch for hvm-3.0-x86_32 guests

This xc_dom_arch will be used in order to build HVM domains. The code is
based on the existing xc_hvm_populate_memory and xc_hvm_populate_params
functions.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@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>