monitor: remove MonitorDef from typedefs.h

monitor: remove MonitorDef from typedefs.h

MonitorDef is defined by hmp-target.h, and all users except one already
include it; the reason why the stubs do not include it, is because
hmp-target.h currently can only be used in files that are compiled
per target.  However, that is easily fixed.  Because the benefit of
having MonitorDef in typedefs.h is very small, do it and remove the
type from typedefs.h.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>