stubs: Move qemu_timer_notify_cb() and remove qemu_notify_event() stub

stubs: Move qemu_timer_notify_cb() and remove qemu_notify_event() stub

When cross-compiling with MinGW, there are sometimes some weird linker
errors like:

ibqemuutil.a(util_main-loop.c.obj): In function `qemu_notify_event':
/builds/huth/qemu/build/../util/main-loop.c:139: multiple definition of
 `qemu_notify_event'
libqemuutil.a(stubs_notify-event.c.obj):/builds/huth/qemu/stubs/notify-event.c:5:
 first defined here
collect2: error: ld returned 1 exit status
/builds/huth/qemu/rules.mak:88: recipe for target 'tests/test-timed-average.exe'
 failed

It seems like it works better when the qemu_timer_notify_cb() stub (which
calls qemu_notify_event()) is in a separate file - then we can also even
remove the qemu_notify_event() stub now.

This patch is based on ideas from the patch "stubs: Remove qemu_notify_event()"
by Philippe Mathieu-Daudé and the patch "cpu-timers, icount: new modules" from
Claudio Fontana.

Message-Id: <20200902102433.304737-1-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>