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

frontend

 Depend on other Qemu subsystem's re-entrant. Currently, the main blockers are memory, interrupt.