Allow the user to select .cdr files in the file open dialog.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 32C964D4-3F17-47B7-AE7E-593E6BFD8855@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>