aio: switch aiocb_size type int -> size_t

Using appropriate types for variables is a good thing :).  All users
simply do sizeof(MyType) and the value is passed to a memory allocator,
it should be size_t.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-aio.h

@@ -23,7 +23,7 @@ typedef void BlockDriverCompletionFunc(void *opaque, int ret);
 
 typedef struct AIOPool {
     void (*cancel)(BlockDriverAIOCB *acb);
-    int aiocb_size;
+    size_t aiocb_size;
     BlockDriverAIOCB *free_aiocb;
 } AIOPool;