qemu-img: Fix qemu-img convert -obacking_file

The old -B option caused a backing file to be used for the converted image and
to avoid copying clusters from the old backing file. When replaced with
-obacking_file, qemu-img convert does assign the backing file to the new image,
but it doesn't realize that it should avoid copying clusters from the backing
file.

This patch checks the -o options for a backing_file and applies the same logic
as for -B in this case.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -645,6 +645,7 @@ static int img_convert(int argc, char **argv)
     const uint8_t *buf1;
     BlockDriverInfo bdi;
     QEMUOptionParameter *param = NULL, *create_options = NULL;
+    QEMUOptionParameter *out_baseimg_param;
     char *options = NULL;
     const char *snapshot_name = NULL;
 
@@ -769,6 +770,12 @@ static int img_convert(int argc, char **argv)
         goto out;
     }
 
+    /* Get backing file name if -o backing_file was used */
+    out_baseimg_param = get_option_parameter(param, BLOCK_OPT_BACKING_FILE);
+    if (out_baseimg_param) {
+        out_baseimg = out_baseimg_param->value.s;
+    }
+
     /* Check if compression is supported */
     if (flags & BLOCK_FLAG_COMPRESS) {
         QEMUOptionParameter *encryption =