Log Message:
- protect Xen's stdarg.h from multiple inclusion.
- build fix for stdarg under NetBSD.

Both backported from upstream. Seems to be needed to build pkg under 5.99.x

http://xenbits.xensource.com/hg/xen-unstable.hg/file/a574bf2f5059/xen/include/xen/stdarg.h