Create storage pool directories with proper uid/gid/mode

Create storage pool directories with proper uid/gid/mode

Previously the uid/gid/mode in the xml was ignored when creating new
storage pool directories. This commit attempts to honor the requested
permissions, and spits out an error if it can't.

Note that when creating the directory, the rest of the path leading up
to the final element is created using current uid/gid/mode, and the
final element gets the settings from xml. It is NOT an error for the
directory to already exist; in this case, the perms for the existing
directory are just set (if necessary).

* src/storage/storage_backend_fs.c: update the virStorageBackendFileSystemBuild
  function to check the directory hierarchy separately then create the
  leaf directory with the right attributes