ui/cocoa.m: Fix updateUIInfo threading issues

ui/cocoa.m: Fix updateUIInfo threading issues

The updateUIInfo method makes Cocoa API calls.  It also calls back
into QEMU functions like dpy_set_ui_info().  To do this safely, we
need to follow two rules:
 * Cocoa API calls are made on the Cocoa UI thread
 * When calling back into QEMU we must hold the iothread lock

Fix the places where we got this wrong, by taking the iothread lock
while executing updateUIInfo, and moving the call in cocoa_switch()
inside the dispatch_async block.

Some of the Cocoa UI methods which call updateUIInfo are invoked as
part of the initial application startup, while we're still doing the
little cross-thread dance described in the comment just above
call_qemu_main().  This meant they were calling back into the QEMU UI
layer before we'd actually finished initializing our display and
registered the DisplayChangeListener, which isn't really valid.  Once
updateUIInfo takes the iothread lock, we no longer get away with
this, because during this startup phase the iothread lock is held by
the QEMU main-loop thread which is waiting for us to finish our
display initialization.  So we must suppress updateUIInfo until
applicationDidFinishLaunching allows the QEMU main-loop thread to
continue.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-id: 20220224101330.967429-2-peter.maydell@linaro.org