Implement support for <hostdev caps=net>

Implement support for <hostdev caps=net>

This allows a container-type domain to have exclusive access to one of
the host's NICs.

Wire <hostdev caps=net> with the lxc_controller - when moving the newly
created veth devices into a new namespace, also look for any hostdev
devices that should be moved. Note: once the container domain has been
destroyed, there is no code that moves the interfaces back to the
original namespace. This does happen, though, probably due to default
cleanup on namespace destruction.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>