vhost-user: qemu stops processing packets under high load of traffic

Description of problem:
- qemu socket becomes full, causing qemu to send incomplete
SET_VRING_CALL messages to vhost-user backend (without proper fd set in
ancillary data).
- after some time, some interrupts are lost, causing the VM to stop
transmitting packets.

How reproducible:
Run a stress tests of a vhost-user interface using an UDP
traffic generator. Traffic generator (IXIA) was connected to 2 physical ports that are in turn connected to 2 virtio ports through a linux bridge, VM
(running linux) doing routing to forward packets between the 2 virtio ports.
When traffic reaches high pps rates of small packets,

Actual results:
- VM stop transmitting packets

Expected results:
- VM should never stop transmitting packets

Additional info:
We do propose a fix at:
  http://lists.nongnu.org/archive/html/qemu-devel/2015-12/msg00652.html