Allow autodev without a rootfs

Allow autodev without a rootfs

A container without a rootfs is useful for running a collection of
processes in separate namespaces (to provide separate networking as
an example), while sharing the host filesystem (except for specific
paths that are re-mounted as needed). For multiple processes to run
automatically when such a container is started, it can be launched
using lxc-start, and a separate instance of systemd can manage just
the processes inside the container. (This assumes that the path to
the systemd unit files is re-mounted and only contains the services
that should run inside the container.) For this use case, autodev
should be permitted for a container that does not have a rootfs.

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>