virsh: Add readonly param in __init__ of VirshSession.

As there is no param about readonly for VirshSession, then we
can not create a readonly Virsh session. This patch add a param
in __init__ of VirshSession class and pass readonly in new_session()
of VirshPersistent. Then we can create a readonly VirshSession and
a readonly VirshPersistent.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>