ui/cocoa: Ensure we have the iothread lock when calling into QEMU

ui/cocoa: Ensure we have the iothread lock when calling into QEMU

The Cocoa UI should run on the main thread; this is enforced
in OSX Mojave. In order to be able to run on the main thread,
we need to make sure we hold the iothread lock whenever we
call into various QEMU UI midlayer functions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-id: 20190225102433.22401-2-peter.maydell@linaro.org
Message-id: 20190214102816.3393-2-peter.maydell@linaro.org