vhost-user is a userspace protocol to establish connectivity
between a virto-net frontend typically qemu and a
userspace virtio backend such as ovs with dpdk.
vhost-user interfaces exist only in userspace from the host perspective
and are not represented in the linux networking stack as kernel netdevs.
As a result attempting to set the mtu on a vhost-user interface
using ifconfig or ip link will fail with a device not found error.
- this change removes a call to _set_device_mtu when plugging
- this change prevents the device not found error from occurring
which stopped vms booting with vhost-user interfaces
due to an uncaught exception resulting in a failure to set the
interface type in ovs.
- this change make creating vhost-user interface
an atomic action.
This latent bug is only triggered when the mtu value is set to a
value other than 0 which was the default proir to mitaka.