qdev: add HotplugHandler->post_plug() callback

qdev: add HotplugHandler->post_plug() callback

The ->pre_plug() callback is invoked before the device is realized.  The
->plug() callback is invoked when the device is being realized but
before it is reset.

This patch adds a ->post_plug() callback which is invoked after the
device has been reset.  This callback is needed by HotplugHandlers that
need to wait until after ->reset().

Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20180716083732[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>