xen: Disallow variable length arrays

xen: Disallow variable length arrays

Variable length arrays result in excess stack utilisation, with a risk
of stack overflow if the length is too large.  It also results in fairly
poor asm generation, because of requiring a divide as part of the space
calcuation.

Xen no longer has any variable length arrays, so take the opportunity to
formally disallow them.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>