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

src/storage/storage_backend.c

@@ -1347,6 +1347,10 @@ virStorageBackendStablePath(virStoragePoolObjPtr pool,
     if (!STRPREFIX(pool->def->target.path, "/dev"))
         goto ret_strdup;
 
+    /* Logical pools are under /dev but already have stable paths */
+    if (pool->def->type == VIR_STORAGE_POOL_LOGICAL)
+        goto ret_strdup;
+
     /* We loop here because /dev/disk/by-{id,path} may not have existed
      * before we started this operation, so we have to give it some time to
      * get created.