virttest.remote: Add return value for handle_prompts

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest.remote: Add connection timeout to handle_prompts

Raise LoginError when get the connection timeout output from session.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1776 from ypu/login

Update handle_prompts in remote.py