ARRAY_SIZE: check that argument is an array

It's a familiar pattern: some code uses ARRAY_SIZE, then refactoring
changes the argument from an array to a pointer to a dynamically
allocated buffer.  Code keeps compiling but any ARRAY_SIZE calls now
return the size of the pointer divided by element size.

Let's add build time checks to ARRAY_SIZE before we allow more
of these in the code-base.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
