Download external tarballs from xenbits.xensource.com

Download external tarballs from xenbits.xensource.com

I have copied the tarballs that the xen-unstable build downloads to
xenbits.xensource.com (which also hosts our hg and git).  This patch
changes the download URLs to use that location.

That way the build will depend on only one external machine, under one
administration, rather than many.  Also it means that the build won't
break if these sites become permanently unavailable or are rearranged
and we don't run a risk of having to panic and beg if a file should go
missing.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>