The virLXCPrepareHostDevices method was returning success even
when it reported an error, and failed to handle several host
device types

This adds support for host device passthrough with the
LXC driver. Since there is only a single kernel image,
it doesn't make sense to pass through PCI devices, but
USB devices are fine. For the latter we merely need to
make the /dev/bus/usb/NNN/MMM character device exist
in the container's /dev
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>