storage: Fix any VolLookupByPath if we have an empty logical pool
storage: Fix any VolLookupByPath if we have an empty logical pool

On F16 at least, empty volume groups don't have a directory under /dev.
The directory only appears once a logical volume is created.

This tickles some behavior in BackendStablePath which ends with
libvirt sleeping for 5 seconds while waiting for the directory to appear.
This causes all sorts of problems for the virStorageVolLookupByPath API
which virtinst uses, even if trying to resolve a path that is independent
of the logical pool.

In reality we don't even need to do that checking since logical pools
always have a stable target path. Short circuit the polling in that
case.

Fixes bug 782261

trainstack/libvirt,libvirt/libvirt,andreabolognani/libvirt,emaste/libvirt,jardasgit/libvirt,novel/fbsd-libvirt,foomango/libvirt,crobinso/libvirt,elmarco/libvirt,danwent/libvirt-ovs,emaste/libvirt,siboulet/libvirt-openvz,zippy2/libvirt,datto/libvirt,wiedi/libvirt,wiedi/libvirt,rmarwaha/libvirt,emaste/libvirt,rmarwaha/libvirt1,agx/libvirt,andreabolognani/libvirt,trainstack/libvirt,zhlcindy/libvirt-1.1.4-maintain,elmarco/libvirt,shugaoye/libvirt,iam-TJ/libvirt,zhlcindy/libvirt-1.1.4-maintain,wiedi/libvirt,rmarwaha/libvirt1,shugaoye/libvirt,foomango/libvirt,bjzhang/libvirt,novel/fbsd-libvirt,novel/fbsd-libvirt,novel/fbsd-libvirt,rlaager/libvirt,nertpinx/libvirt,andreabolognani/libvirt,wiedi/libvirt,cbosdo/libvirt,taget/libvirt,olafhering/libvirt,eskultety/libvirt,bjzhang/libvirt,jeckersb/libvirt,dumbbell/libvirt,iam-TJ/libvirt,cbosdo/libvirt,dumbbell/libvirt,dumbbell/libvirt,zhlcindy/libvirt-1.1.4-maintain,emaste/libvirt,trainstack/libvirt,siboulet/libvirt-openvz,bjzhang/libvirt,eskultety/libvirt,rlaager/libvirt,fabianfreyer/libvirt,novel/fbsd-libvirt,agx/libvirt,rmarwaha/libvirt1,VenkatDatta/libvirt,emaste/libvirt,bjzhang/libvirt,siboulet/libvirt-openvz,fabianfreyer/libvirt,warewolf/libvirt,foomango/libvirt,andreabolognani/libvirt,datto/libvirt,jfehlig/libvirt,cbosdo/libvirt,danwent/libvirt-ovs,datto/libvirt,trainstack/libvirt,zippy2/libvirt,warewolf/libvirt,zippy2/libvirt,wiedi/libvirt,jfehlig/libvirt,iam-TJ/libvirt,jfehlig/libvirt,rmarwaha/libvirt1,crobinso/libvirt,warewolf/libvirt,emaste/libvirt,dumbbell/libvirt,VenkatDatta/libvirt,datto/libvirt,taget/libvirt,jardasgit/libvirt,bjzhang/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,danwent/libvirt-ovs,rmarwaha/libvirt1,nertpinx/libvirt,wiedi/libvirt,novel/fbsd-libvirt,andreabolognani/libvirt,elmarco/libvirt,jardasgit/libvirt,foomango/libvirt,rmarwaha/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,danwent/libvirt-ovs,eskultety/libvirt,olafhering/libvirt,iam-TJ/libvirt,rmarwaha/libvirt1,elmarco/libvirt,novel/fbsd-libvirt,foomango/libvirt,zhlcindy/libvirt-1.1.4-maintain,trainstack/libvirt,warewolf/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,libvirt/libvirt,iam-TJ/libvirt,taget/libvirt,rmarwaha/libvirt,warewolf/libvirt,VenkatDatta/libvirt,rmarwaha/libvirt,novel/fbsd-libvirt,rlaager/libvirt,zippy2/libvirt,fabianfreyer/libvirt,eskultety/libvirt,siboulet/libvirt-openvz,shugaoye/libvirt,jardasgit/libvirt,rlaager/libvirt,agx/libvirt,crobinso/libvirt,trainstack/libvirt,dumbbell/libvirt,emaste/libvirt,dumbbell/libvirt,rlaager/libvirt,olafhering/libvirt,jardasgit/libvirt,jeckersb/libvirt,warewolf/libvirt,eskultety/libvirt,nertpinx/libvirt,siboulet/libvirt-openvz,danwent/libvirt-ovs,datto/libvirt,crobinso/libvirt,novel/fbsd-libvirt,jeckersb/libvirt,taget/libvirt,jeckersb/libvirt,rmarwaha/libvirt,wiedi/libvirt,jeckersb/libvirt,cbosdo/libvirt,rmarwaha/libvirt,jeckersb/libvirt,warewolf/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,taget/libvirt,trainstack/libvirt,agx/libvirt,nertpinx/libvirt,nertpinx/libvirt,shugaoye/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,agx/libvirt,jeckersb/libvirt,iam-TJ/libvirt,iam-TJ/libvirt,VenkatDatta/libvirt