virt: Add SSH connection into utils_conn module.

Add a class named SSHConnection extending Connection.
This class will setup a connection from client to server
by ssh using authentication of rsa key.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
virt: using SSHConnection in utils_conn to setup a connection from local to remote.

Call check method to check connection from local to remote.
If fail, call the setup method and check again.
If OK, skip setup.

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