viralloc: Remove VIR_ALLOC_VAR

viralloc: Remove VIR_ALLOC_VAR

The use case VIR_ALLOC_VAR deals with is very unlikely. We had just 2
legitimate uses, which were reimplemented locally using g_malloc0 and
sizeof instead as they used a static number of members of the trailing
array.

Remove VIR_ALLOC_VAR since in most cases the direct implementation is
shorter and clearer and there are no users of it currently.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>