Currently to make sysfs readonly, we remount the existing
instance and then bind it readonly. Unfortunately this means
sysfs is still showing device objects wrt the host OS namespace.
We need it to reflect the container namespace, so we must mount
a completely new instance of it. Do the same for selinuxfs since
there is no benefit to bind mounting & this lets us simplify
the code.

* src/lxc/lxc_container.c: Mount fresh sysfs instance
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>