support for QEMU vhost-user

support for QEMU vhost-user

This patch adds support for the QEMU vhost-user feature to libvirt.
vhost-user enables the communication between a QEMU virtual machine
and other userspace process using the Virtio transport protocol.
It uses a char dev (e.g. Unix socket) for the control plane,
while the data plane based on shared memory.

The XML looks like:

<interface type='vhostuser'>
    <mac address='52:54:00:3b:83:1a'/>
    <source type='unix' path='/tmp/vhost.sock' mode='server'/>
    <model type='virtio'/>
</interface>

Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>