ui: Fix hanging up Cocoa display on macOS 10.15 (Catalina)

ui: Fix hanging up Cocoa display on macOS 10.15 (Catalina)

macOS API documentation says that before applicationDidFinishLaunching
is called, any events will not be processed. However, some events are
fired before it is called in macOS Catalina. This causes deadlock of
iothread_lock in handleEvent while it will be released after the
app_started_sem is posted.
This patch avoids processing events before the app_started_sem is
posted to prevent this deadlock.

Buglink: https://bugs.launchpad.net/qemu/+bug/1847906
Signed-off-by: Hikaru Nishida <hikarupsp@gmail.com>
Message-id: 20191015010734.85229-1-hikarupsp@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit dff742ad27efa474ec04accdbf422c9acfd3e30e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>