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)

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.