Specially handle block device rootfs

It is not possible to mount a block device from a non-init user namespace.
Therefore if root on the host is starting a container with a uid
mapping, and the rootfs is a block device, then mount the rootfs before
we spawn the container init task.

This addresses https://github.com/lxc/lxc/issues/221

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>