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