Fix unix-char/unix-block in nested containers
Fix unix-char/unix-block in nested containers

This comes with a number of limitations due to us not being able to
mknod the device, but it's better than what we have currently.

Closes #2279

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

mjeanson/lxd,stgraber/lxd,stgraber/lxd,erikmack/lxd,georghopp/lxd,chrisglass/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,akshaykarle/lxd,erikmack/lxd,pcdummy/lxd,pcdummy/lxd,hallyn/lxd,ollie314/lxd,stgraber/lxd,lxc/lxd,Skarlso/lxd,pcdummy/lxd,hallyn/lxd,jsavikko/lxd,atxwebs/lxd,chrisglass/lxd,lxc/lxd,stgraber/lxd,akshaykarle/lxd,georghopp/lxd,hallyn/lxd,hallyn/lxd,Skarlso/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,georghopp/lxd,mjeanson/lxd,techtonik/lxd,Skarlso/lxd,techtonik/lxd,chrisglass/lxd,akshaykarle/lxd,stgraber/lxd,lxc/lxd,techtonik/lxd,lxc/lxd,jsavikko/lxd,Skarlso/lxd,mjeanson/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,erikmack/lxd,jsavikko/lxd,lxc/lxd,ollie314/lxd,ollie314/lxd