Vhost/Virtio Fixes

This series include a variety of fixes:
 - wrong definition of pull() in net_device (probably merge issue)
 - use timer to process the vhost-user QEMU connection. Set top 500ms, this ensures that the connection is processed independent of push/pull frequency
 - optimize address mapping - not use return in the middle of for (as suggested for performance reasons)
 - suppress events (interrupts) - implement the simple mechanism described in the virtio spec
 - in bench - replace wait with sleep and then kill them, avoids test hangs