libxl: check nesthvm and altp2m in libxl

libxl: check nesthvm and altp2m in libxl

In ea214001 ("x86/altp2m: add altp2mhvm HVM domain parameter"), a
check was added to ensure nestedhvm and altp2m cannot be enabled at
the same time. That check was added in xl, but in fact it should be in
libxl because it should be the entity that decides whether
the provided configuration is valid.

This patch moves the check to libxl. The code snippet is moved after
calling libxl__domain_build_info_setdefault so that we can:
1. remove libxl_defbool_is_default in `if()';
2. detect mistake in libxl__domain_build_info_setdefault.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>