honor lxc.rootfs.bdev
honor lxc.rootfs.bdev

If that is specified, then we only use the specified backing store type.

This can be useful if you know that lxc.rootfs is a directory type and
you do not want lxc to waste time searching for zfs, btrfs, etc.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

caglar10ur/lxc,thmo/lxc,hfuCN/lxc,federicobriata/lxc,ysbnim/lxc,thmo/lxc,hfuCN/lxc,thmo/lxc,hfuCN/lxc,LynxChaus/lxc,hallyn/lxc,terceiro/lxc,GreatFruitOmsk/lxc,caglar10ur/lxc,federicobriata/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,cjwatson/lxc,ysbnim/lxc,ss1h2a3tw/lxc,QingweiPeterLan/cs188-lxc,GreatFruitOmsk/lxc,terceiro/lxc,hallyn/lxc,hallyn/lxc,terceiro/lxc,akshaykarle/lxc,aeris/lxc,QingweiPeterLan/cs188-lxc,ysbnim/lxc,caglar10ur/lxc,akshaykarle/lxc,ss1h2a3tw/lxc,cjwatson/lxc,federicobriata/lxc,LynxChaus/lxc,cjwatson/lxc,QingweiPeterLan/cs188-lxc,GreatFruitOmsk/lxc,aeris/lxc,akshaykarle/lxc,aeris/lxc