Align to latest updates in vhost-user implementation in QEMU 2.1

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Add vhost_user_listen

This function will be used to establish a listening server for vhost-user.
Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
VhostUser enable server mode

The class will be able to run as a vhost-user server waiting for connection.
The new method will get args with socket_path and is_server.

The vhost_user selftest can be started with SNABB_TEST_VHOST_USER_SERVER to enable server mode.

All VhosUser:new callers are updated to reflect the args change.

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
NFV design, run infinite if number of packets is 0

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Merge pull request #225 from virtualopensystems/align-qemu-2.1

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