rootfs_is_blockdev: don't run if no rootfs is specified
rootfs_is_blockdev: don't run if no rootfs is specified

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

raspberrypython/lxc,ksperis/lxc,bmoar/lxc,efiop/lxc,christiaan/lxc,hfuCN/lxc,rldleblanc/lxc,Distrotech/lxc,ioops/lxc,armcc/lxc,fwilson42/lxc,rldleblanc/lxc,QingweiPeterLan/cs188-lxc,jirislaby/lxc,jirislaby/lxc,regit/lxc,regit/lxc,hallyn/lxc,federicobriata/lxc,JohnPeacockMessageSystems/lxc,DarknessBeforeDawn/lxc,weizhenwei/lxc,Distrotech/lxc,weizhenwei/lxc,DarknessBeforeDawn/lxc,GreatFruitOmsk/lxc,fwilson42/lxc,thmo/lxc,ss1h2a3tw/lxc,virajs/lxc,major/lxc,cjwatson/lxc,LynxChaus/lxc,szaszg/lxc,ksperis/lxc,hallyn/lxc,virajs/lxc,terceiro/lxc,GreatFruitOmsk/lxc,caglar10ur/lxc,bostjan/lxc,thmo/lxc,aeris/lxc,cjwatson/lxc,yubo/lxc,cjwatson/lxc,federicobriata/lxc,hkjolhede/lxc,LynxChaus/lxc,aeris/lxc,terceiro/lxc,thmo/lxc,rowhit/lxc,ecarrara/lxc,sapun/lxc,yubo/lxc,efiop/lxc,hfuCN/lxc,regit/lxc,terceiro/lxc,hfuCN/lxc,Pelagicore/lxc,Pelagicore/lxc,JohnPeacockMessageSystems/lxc,Distrotech/lxc,tych0/lxc,bostjan/lxc,efiop/lxc,sapun/lxc,hkjolhede/lxc,rowhit/lxc,fwilson42/lxc,DarknessBeforeDawn/lxc,szaszg/lxc,jirislaby/lxc,JohnPeacockMessageSystems/lxc,weizhenwei/lxc,lazy404/lxc,rldleblanc/lxc,szaszg/lxc,lisongmin/lxc,aeris/lxc,akshaykarle/lxc,Pelagicore/lxc,ioops/lxc,armcc/lxc,bostjan/lxc,lisongmin/lxc,LynxChaus/lxc,hkjolhede/lxc,akshaykarle/lxc,QingweiPeterLan/cs188-lxc,ss1h2a3tw/lxc,codebauss/lxc,major/lxc,christiaan/lxc,rowhit/lxc,codebauss/lxc,armcc/lxc,ecarrara/lxc,bmoar/lxc,ysbnim/lxc,GreatFruitOmsk/lxc,lazy404/lxc,QingweiPeterLan/cs188-lxc,ysbnim/lxc,raspberrypython/lxc,tych0/lxc,ss1h2a3tw/lxc,ksperis/lxc,Ponce/lxc,federicobriata/lxc,tych0/lxc,Ponce/lxc,bmoar/lxc,sapun/lxc,major/lxc,ioops/lxc,caglar10ur/lxc,virajs/lxc,codebauss/lxc,ecarrara/lxc,akshaykarle/lxc,ysbnim/lxc,lisongmin/lxc,caglar10ur/lxc,hallyn/lxc,lazy404/lxc,raspberrypython/lxc,christiaan/lxc,Ponce/lxc,yubo/lxc