tests/vhost-user-bridge: add vhost-user bridge application

The test existing in QEMU for vhost-user feature is good for
testing the management protocol, but does not allow actual
traffic. This patch proposes Vhost-User Bridge application, which
can serve the QEMU community as a comprehensive test by running
real internet traffic by means of vhost-user interface.

Essentially the Vhost-User Bridge is a very basic vhost-user
backend for QEMU. It runs as a standalone user-level process.
For packet processing Vhost-User Bridge uses an additional QEMU
instance with a backend configured by "-net socket" as a shared
VLAN.  This way another QEMU virtual machine can effectively
serve as a shared bus by means of UDP communication.

For a more simple setup, the another QEMU instance running the
SLiRP backend can be the same QEMU instance running vhost-user
client.

This Vhost-User Bridge implementation is very preliminary.  It is
missing many features. I has been studying vhost-user protocol
internals, so I've written vhost-user-bridge bit by bit as I
progressed through the protocol.  Most probably its internal
architecture will change significantly.

To run Vhost-User Bridge application:

1. Build vhost-user-bridge with a regular procedure. This will
create a vhost-user-bridge executable under tests directory:

    $ configure; make tests/vhost-user-bridge

2. Ensure the machine has hugepages enabled in kernel with
command line like:

    default_hugepagesz=2M hugepagesz=2M hugepages=2048

3. Run Vhost-User Bridge with:

    $ tests/vhost-user-bridge

The above will run vhost-user server listening for connections
on UNIX domain socket /tmp/vubr.sock, and will try to connect
by UDP to VLAN bridge to localhost:5555, while listening on
localhost:4444

Run qemu with a virtio-net backed by vhost-user:

    $ qemu \
        -enable-kvm -m 512 -smp 2 \
        -object memory-backend-file,id=mem,size=512M,mem-path=/dev/hugepages,share=on \
        -numa node,memdev=mem -mem-prealloc \
        -chardev socket,id=char0,path=/tmp/vubr.sock \
        -netdev type=vhost-user,id=mynet1,chardev=char0,vhostforce \
        -device virtio-net-pci,netdev=mynet1 \
        -net none \
        -net socket,vlan=0,udp=localhost:4444,localaddr=localhost:5555 \
        -net user,vlan=0 \
        disk.img

vhost-user-bridge was tested very lightly: it's able to bringup a
linux on client VM with the virtio-net driver, and execute transmits
and receives to the internet. I tested with "wget redhat.com",
"dig redhat.com".

PS. I've consulted DPDK's code for vhost-user during Vhost-User
Bridge implementation.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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