qemu-img/convert: Always set ret < 0 on error

qemu-img/convert: Always set ret < 0 on error

Otherwise the qemu-img process will exit with EXIT_SUCCESS instead of
EXIT_FAILURE.

Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
* applied directly to stable, upstream code has issue fixed via a
  refactoring introduced by 9fd77f9, which isn't targetted for stable
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>