ui/cocoa: Do not exit immediately after shutdown

ui/cocoa used to call exit immediately after calling
qemu_system_shutdown_request, which prevents QEMU from actually
perfoming system shutdown. Just sleep forever, and wait QEMU to call
exit and kill the Cocoa thread.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20210219111652.20623-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>