libvirt-virsh: Add remote_pwd into VirshConnectBack.__slots__.

If 'remote_pwd' is not in __slots__, we can not pass a remote_pwd
into VirshConnectBack, so that we can not connect to a remote host
with password. Same reason to 'remote_user'.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>