Features/network reentrant

benefit of network layer re-entrant

 make the component of network run on multi-thread
 with glib, it is easy to do unit test

network layer re-entrant includes

 re-entrant of network core 
 re-entrant of backend like tap
 re-entrant of frontend like virtio net

network core

 The thread safety issue focus on NetClientState, NetQueue and hub. 
 (proposal patches have been sent out. See:[PATCH v2 0/5] make netlayer re-entrant [1])

backend

 make glib as the event loop for backends.
 consider about the race between the iohandler and NetClientInfo callback
 (make tap as the first example)

frontend

 Depend on other Qemu subsystem's re-entrant. Currently, the main blockers are memory, interrupt.
 (make virtio net as the first example)

glib OR AioContext

disadvantage for current AioContext

 1st. need to define and expand interface for other fd events, while glib open this interface for user *
 2nd. need to add support for IOCanReadHandler, while gsource provide prepare, check method to allow more flexible control
 3rd. block layer's AioContext will block other AioContexts on the same thread.
 4th. need more document

disadvantage for glib

 1st. need re-implement fd's handlers under the same GSource (aio_set_file_handler)