Align QEMU 2.1, add VhostUser server and infinite mode in NFV design

This pull request adds the following major changes:

 - align with latest QEMU 2.1 changes for vhost-user (updated in https://github.com/virtualopensystems/qemu/tree/large-vring)
 
 - NFV design will not exit of npackets is 0 (useful with bench environment)

 - VhostUser can run as server and updated all users of this class