Features/network reentrant

The network layer re-entrant includes the following efforts:

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

network core

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

backend

 using glib as the event loop to make them thread safe

frontend

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