monitor: remove event_clock_type

Instead, use a dynamic function to detect which clock we'll use.  The
problem is that the old code will let monitor initialization depend on
configure_accelerator() (that's where qtest_enabled() start to take
effect).  After this change, we don't have such a dependency any more.
We just need to make sure configure_accelerator() is called when we
start to use it.  Now it's only used in monitor_qapi_event_queue() and
monitor_qapi_event_handler(), so we're good.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180608035511.7439-6-peterx@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[monitor_get_event_clock() name and comment tweaked]
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>