Reorganizes network frontend device unregister and removal, so that

Reorganizes network frontend device unregister and removal, so that
occur in the right order and succeed.

Now on Closing state we:
- stop queue
- unbind irq
- unregister the device

On removal:
- end foreign access to TX and RX pages (we cannot do this before)
- free_netdev, doing the last stage of destroying an allocated device

Makes xm-test network "03_attach_detach_multiple_pos" pass:
http://xmtest.dague.org/cgi-bin/display?view=single&testid=1068

Signed-off-by: Murillo F. Bernardes <mfb@br.ibm.com>