block: Introduce bs->zero_beyond_eof

In 4146b46c42e0989cb5842e04d88ab6ccb1713a48 (block: Produce zeros when
protocols reading beyond end of file), we break qemu-iotests ./check
-qcow2 022. This happens because qcow2 temporarily sets ->growable = 1
for vmstate accesses (which are stored beyond the end of regular image
data).

We introduce the bs->zero_beyond_eof to allow qcow2_load_vmstate() to
disable ->zero_beyond_eof temporarily in addition to enable ->growable.

[Since the broken patch "block: Produce zeros when protocols reading
beyond end of file" has not been merged yet, I have applied this fix
*first* and will then apply the next patch to keep the tree bisectable.
-- Stefan]

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/block_int.h

@@ -281,6 +281,9 @@ struct BlockDriverState {
     /* Whether the disk can expand beyond total_sectors */
     int growable;
 
+    /* Whether produces zeros when read beyond eof */
+    bool zero_beyond_eof;
+
     /* the memory alignment required for the buffers handled by this driver */
     int buffer_alignment;