Description of problem:
When trying to launch a VM, virDomainDefineXML removes "mode" attribute from a vhostuser interface, causing domain.createWithFlags to fail in nova/virt/libvirt/driver.py .
According to libvirt website, vhost-user should have been supported since 1.2.7 (https://libvirt.org/formatdomain.html#elementVhostuser).

Version-Release number of selected component (if applicable):
libvirt is 1.2.9-9

How reproducible:
I am working with openstack kilo, where libvirt is set to 1.2.9-9, with opendaylight as the controller, but I believe this can be reproduced using devstack kilo with networking-ovs-dpdk plugin.

Steps to Reproduce:
1. load devstack as allinone node, with networking-ovs-dpdk enabled
2. launch a VM
3. make sure libvirt is set to 1.2.9

Actual results:
libvirtError: internal error: No <source> 'mode' attribute specified with <interface type='vhostuser'/>

Expected results:
VM instantiation succeeds

Additional info: