libxl: fix libxl_domain_need_memory after 899433f149d

libxl: fix libxl_domain_need_memory after 899433f149d

After 899433f149d libxl needs to know the content of d_config to
determine which QEMU is used. The code is changed such that
libxl__domain_set_device_model needs to be called before
libxl__domain_build_info_setdefault.

This is fine for libxl code, but it is problematic for
libxl_domain_need_memory, which is the only public API that takes a
build_info. To avoid breaking its users, provide a compatibility
setting inside that function.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>