Commit 8b55992f added some Coverity comments to silence what was
a real bug in the code.  Since then, we've had a miserable run
of trying to fix the underlying problem (commits c059cdea and
ba5193c8), and still have a problem on 32-bit machines.

This fixes the problem for once and for all, by realizing that
on older xen, cpumap_t is identical to uint64_t, and using the
new virendian.h to do the transformation from the API (documented
to be little-endian) to the host structure.

* src/xen/xen_hypervisor.c (virXen_setvcpumap): Do the conversion
correctly.  Finally.