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.

Signed-off-by: Feng Yang <fyang@redhat.com>