tools: make overlay valid backend
tools: make overlay valid backend

So far, users could only create overlay snapshots by specifying -B overlayfs
and not with -B overlay. This adds support for -B overlay.

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

GreatFruitOmsk/lxc,hallyn/lxc,ysbnim/lxc,hfuCN/lxc,GreatFruitOmsk/lxc,LynxChaus/lxc,federicobriata/lxc,thmo/lxc,hfuCN/lxc,cjwatson/lxc,thmo/lxc,hallyn/lxc,cjwatson/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,ysbnim/lxc,aeris/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,terceiro/lxc,terceiro/lxc,hfuCN/lxc,thmo/lxc,LynxChaus/lxc,LynxChaus/lxc,federicobriata/lxc,ysbnim/lxc,caglar10ur/lxc,federicobriata/lxc,ss1h2a3tw/lxc,hallyn/lxc,aeris/lxc,terceiro/lxc,GreatFruitOmsk/lxc,cjwatson/lxc,aeris/lxc