virttest.remote: Update password and login prompt in handle_prompts function

    some kernel msg makes autotest confusing:
    "Password: mtrr: type mismatch for c0000000,1000000 old: uncachable new: write-combining"
    
    Then we may got error
    "Unhandled LoginAuthenticationError: Got username prompt twice"
    
    This update just want to make password and login prompt still works in this situation.