qemu-img/convert: Move bs_n > 1 && -B check down

It does not make much sense to use a backing image for the target when
you concatenate multiple images (because then there is no correspondence
between the source images' backing files and the target's); but it was
still possible to give one by using -o backing_file=X instead of -B X.

Fix this by moving the check.

(Also, change the error message because -B is not the only way to
 specify the backing file, evidently.)

Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
* applied patch from v1 of series as suggested by author
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qemu-img.c

@@ -2113,13 +2113,6 @@ static int img_convert(int argc, char **argv)
     }
 
 
-    if (bs_n > 1 && out_baseimg) {
-        error_report("-B makes no sense when concatenating multiple input "
-                     "images");
-        ret = -1;
-        goto out;
-    }
-
     src_flags = 0;
     ret = bdrv_parse_cache_mode(src_cache, &src_flags, &src_writethrough);
     if (ret < 0) {
@@ -2229,6 +2222,13 @@ static int img_convert(int argc, char **argv)
         out_baseimg = out_baseimg_param;
     }
 
+    if (bs_n > 1 && out_baseimg) {
+        error_report("Having a backing file for the target makes no sense when "
+                     "concatenating multiple input images");
+        ret = -1;
+        goto out;
+    }
+
     /* Check if compression is supported */
     if (compress) {
         bool encryption =

tests/qemu-iotests/122.out

@@ -61,8 +61,8 @@ read 65536/65536 bytes at offset 4194304
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 65536/65536 bytes at offset 8388608
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-qemu-img: -B makes no sense when concatenating multiple input images
-qemu-img: -B makes no sense when concatenating multiple input images
+qemu-img: Having a backing file for the target makes no sense when concatenating multiple input images
+qemu-img: Having a backing file for the target makes no sense when concatenating multiple input images
 
 === Compression with misaligned allocations and image sizes ===