Find free device during host startup.
Find free device during host startup.

In some cases original loop device is busy before it is losetup-ed to serve for VG. Added logic to find free one if this happens.
Fixes: rhbz#971145
Change-Id: I4df844f85aa31996bc25fb8539614135bb70d57f

spkane/packstack,xbezdick/packstack,mahak/packstack,pkilambi/packstack,fr34k8/packstack,weshayutin/packstack,palmer159/openstack-test,alincalinciuc/packstack,weshayutin/packstack,harveyzh/packstack,larsks/packstack,larsks/packstack,palmer159/openstack-test,alincalinciuc/packstack,stackforge/packstack,spkane/packstack,radez/packstack,imcsk8/packstack,harveyzh/packstack,paramite/packstack,fr34k8/packstack,mahak/packstack,radez/packstack,spkane/packstack,radez/packstack,stackforge/packstack,paramite/packstack,imcsk8/packstack,pkilambi/packstack,mahak/packstack,openstack/packstack,larsks/packstack,pkilambi/packstack,bcrochet/packstack,bcrochet/packstack,bcrochet/packstack,harveyzh/packstack,openstack/packstack,fr34k8/packstack,paramite/packstack,weshayutin/packstack,xbezdick/packstack,imcsk8/packstack,alincalinciuc/packstack,palmer159/openstack-test,stackforge/packstack,xbezdick/packstack,openstack/packstack