public: add macro for defining variable length array in public headers

public: add macro for defining variable length array in public headers

Several public headers of the hypervisor contain structures with
variable length arrays. In order to be usable with different compilers
those definitions are depending on the compiler type and the standard
supported by the compiler.

In order to avoid open coding the different variants in each header
add a common macro for that purpose in xen.h.

This at once corrects most of the definitions which miss one case
leading to not defining the array at all.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>