Only ummount /proc, /sys, /dev if the root source is '/'

Only ummount /proc, /sys, /dev if the root source is '/'

Previous commits added code to unmount the existing /proc,
/sys and /dev hierarchies on the root filesystem of the
container. This should only have been done if the container's
root filesystem was the same as the host's root. ie if
the root source is '/'.   As it is, this causes LXC containersr
to fail to start if their root source is not '/'

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>