ui/cocoa.m: add toast file support

Add the ability for the user to use .toast files with QEMU. This format works
just like ISO files.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 0C9DA454-E3DC-4291-806E-9A96557DE833@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>