vhost-user support: domain configuration

vhost-user is added as a virDomainChrSourceDefPtr variable in the
virtual network interface configuration structure.
This patch adds support to parse vhost-user element. The XML file
looks like:

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

Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>
vhost-user support: qemu command-line

Build the qemu-command line for vhost-user.

Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>
vhost-user support: tests and docs

This patch adds the test files and the documentation for vhost-user.

Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>
vhost-user support: lxc,xenxs,uml

vhost-user is a qemu feature. Initial support for lxc,uml and
xenxs added.

Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>