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.

glib OR AioContext

 Expand G_IO_IN in AioContext, or leave a gap for user to provide dispatcher
 Add full support for IOCanReadHandler
 For sync reason, block layer's AioContext will block the other AioContext's dispather