qemu-img: convert: don't use unallocated_blocks_are_zero

qemu-img convert wants to distinguish ZERO which comes from short
backing files. unallocated_blocks_are_zero field of bdi is unrelated:
space after EOF is always considered to be zero anyway. So, just make
post_backing_zero true in case of short backing file.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200528094405.145708-2-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

qemu-img.c

@@ -1681,7 +1681,6 @@ typedef struct ImgConvertState {
     BlockBackend *target;
     bool has_zero_init;
     bool compressed;
-    bool unallocated_blocks_are_zero;
     bool target_is_new;
     bool target_has_backing;
     int64_t target_backing_sectors; /* negative if unknown */
@@ -1726,7 +1725,7 @@ static int convert_iteration_sectors(ImgConvertState *s, int64_t sector_num)
 
     if (s->target_backing_sectors >= 0) {
         if (sector_num >= s->target_backing_sectors) {
-            post_backing_zero = s->unallocated_blocks_are_zero;
+            post_backing_zero = true;
         } else if (sector_num + n > s->target_backing_sectors) {
             /* Split requests around target_backing_sectors (because
              * starting from there, zeros are handled differently) */
@@ -2678,7 +2677,6 @@ static int img_convert(int argc, char **argv)
     } else {
         s.compressed = s.compressed || bdi.needs_compressed_writes;
         s.cluster_sectors = bdi.cluster_size / BDRV_SECTOR_SIZE;
-        s.unallocated_blocks_are_zero = bdi.unallocated_blocks_are_zero;
     }
 
     ret = convert_do_copy(&s);