hyper-v: trace vmbus_on_message()

Add tracepoint to vmbus_on_message() which is called when we start
processing a blocking from work context.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>