Install Errors on Version 12:
This error happened on QEMU emulator version 2.11.1 pve-qemu-kvm_2.11.1-5
on Proxmox/Debian but installing on QEMU.12 on Centos 6 did not produce the error.
*Update it is not related to the OS or QEMU version. This happened in Centos 6 too after a second install.
What really causes this even though you successfully install........
In Debian a lot of times SSH disables the root user to login by password by default. This means you will get an authentication failure as if you typed in the wrong password.
The logs also indicate the password is wrong but what is often the case is in the config file
cat /etc/ssh/sshd_config|grep -i permitrootlogin
Make sure it says:
If not change it and restart SSH........