libxl: Use libxl_strdup instead of strdup on libxl_version_info

libxl: Use libxl_strdup instead of strdup on libxl_version_info

The change is simple replace of raw strdup with a libxl variant.
The benefit of that is the libxl variant has the extra
behaviour of abort-on-alloc-fail - and will improve error handling.

libxl_version_info is a bit odd - it is a public function and as libxl.h
mentions - the callers of libxl_ public function needs to call the appropiate
_dispose() function.

"However libxl_get_version_info() is special and returns a cached
result from the ctx which cannot and should not be freed (as evidenced
by it returning a const struct). This data is freed in libxl_ctx_free()
by calling libxl_version_info_dispose(). This is why none of the callers
remember to free -- they shouldn't be doing so." (Ian Campbell)

So the patch makes sure to use the NOGC.

Suggested-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>