container_lxc: check whether disk device exists
container_lxc: check whether disk device exists

We should check whether the disk device exists under the containers disk device
entries and only then try to forkumount.

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

mjeanson/lxd,Skarlso/lxd,stgraber/lxd,erikmack/lxd,stgraber/lxd,Skarlso/lxd,stgraber/lxd,mjeanson/lxd,Skarlso/lxd,hallyn/lxd,pcdummy/lxd,pcdummy/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,techtonik/lxd,jsavikko/lxd,lxc/lxd,erikmack/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd,erikmack/lxd,lxc/lxd,stgraber/lxd,techtonik/lxd,lxc/lxd,techtonik/lxd,pcdummy/lxd,hallyn/lxd,mjeanson/lxd,jsavikko/lxd,stgraber/lxd,lxc/lxd,jsavikko/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,atxwebs/lxd