container_lxc: fix optional property for disk devs
container_lxc: fix optional property for disk devs

Closes #4538.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

lxc/lxd,atxwebs/lxd,hallyn/lxd,atxwebs/lxd,Skarlso/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,Skarlso/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,mjeanson/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,erikmack/lxd,stgraber/lxd,hallyn/lxd,erikmack/lxd,stgraber/lxd,erikmack/lxd,mjeanson/lxd,mjeanson/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd