avocado-vt testing qcow2 imgae with error “ login timeout expired “

I use avocado-VT to test myself debian based .qcow2 file ,just obtained a login timeout expired error,but it work well when testing the default jeos image. it breaks in function wait_for_login, Besides . I notice that the VT use a ssh root@127.0.0.1 -p 5000 to connect to guest-os, I wonder if it is correct, or any settings need to be setted to make it go smootly? thank!