iotests: Use stat -c %b in 125

125 should not use qemu-img to get the on-disk image size, because that
reports it in a human-readable format that is useless to us.  Just use
stat instead (like we do to get the image file length).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190925183231.11196-4-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/125

@@ -34,8 +34,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 
 get_image_size_on_host()
 {
-    $QEMU_IMG info -f "$IMGFMT" "$TEST_IMG" | grep "disk size" \
-        | sed -e 's/^[^0-9]*\([0-9]\+\).*$/\1/'
+    echo $(($(stat -c '%b * %B' "$TEST_IMG_FILE")))
 }
 
 # get standard environment and filters