hvmloader: use xen/errno.h rather than the host systems errno.h

hvmloader: use xen/errno.h rather than the host systems errno.h

hvmloader is unhosted, and shouldn't use the system errno.h.  It already has
to use Xen's errno.h for other hypercalls.  The use of public/io/xs_wire.h
requires the use of un-prefixed errno values.

This fixes the build on stricter toolchains where requesting -fno-builtin does
reduce the include path as much as it can.

Reported-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>