make error codes a formal part of the ABI

make error codes a formal part of the ABI

Now that we have two cases where patches against hvmloader got
submitted needing to include the hypervisor's errno.h (for the host's
system header not necessarily reflecting the correct numbers), take
this as a strong sign that we need to make the error return values part
of the hypervisor ABI (which de-fact they've always been).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>