spec: add mock for pool.persistent?

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
README: explain how to migrate on vagrant 1.5.x

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
README: add a way to remove pool with virt-manager GUI

Signed-off-by: Hiroshi Miura <miurahr@linux.com>