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>

ui/cocoa.m

@@ -879,7 +879,7 @@ QemuCocoaView *cocoaView;
         // set the supported image file types that can be opened
         supportedImageFileTypes = [NSArray arrayWithObjects: @"img", @"iso", @"dmg",
                                  @"qcow", @"qcow2", @"cloop", @"vmdk", @"cdr",
-                                  nil];
+                                  @"toast", nil];
         [self make_about_window];
     }
     return self;