storage: qemu-img: change INFO to DEBUG
storage: qemu-img: change INFO to DEBUG

For really old qemu-img binaries which do not support specifying
the format of the backing file, display a DEBUG message instead of
INFO that this can't be done.

nertpinx/libvirt,datto/libvirt,cbosdo/libvirt,shugaoye/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,cbosdo/libvirt,zippy2/libvirt,elmarco/libvirt,andreabolognani/libvirt,cbosdo/libvirt,eskultety/libvirt,agx/libvirt,shugaoye/libvirt,eskultety/libvirt,fabianfreyer/libvirt,elmarco/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,elmarco/libvirt,andreabolognani/libvirt,rlaager/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,siboulet/libvirt-openvz,shugaoye/libvirt,elmarco/libvirt,eskultety/libvirt,iam-TJ/libvirt,fabianfreyer/libvirt,iam-TJ/libvirt,crobinso/libvirt,siboulet/libvirt-openvz,crobinso/libvirt,datto/libvirt,cbosdo/libvirt,cbosdo/libvirt,libvirt/libvirt,iam-TJ/libvirt,agx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,agx/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,datto/libvirt,jfehlig/libvirt,libvirt/libvirt,shugaoye/libvirt,agx/libvirt,rlaager/libvirt,elmarco/libvirt,olafhering/libvirt,taget/libvirt,zippy2/libvirt,olafhering/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,olafhering/libvirt,iam-TJ/libvirt,datto/libvirt,datto/libvirt,jfehlig/libvirt,libvirt/libvirt,taget/libvirt,zippy2/libvirt,siboulet/libvirt-openvz,VenkatDatta/libvirt,rlaager/libvirt,iam-TJ/libvirt,siboulet/libvirt-openvz,eskultety/libvirt,VenkatDatta/libvirt,siboulet/libvirt-openvz,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,iam-TJ/libvirt,rlaager/libvirt,rlaager/libvirt,crobinso/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,iam-TJ/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,agx/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,taget/libvirt,taget/libvirt