Inicie outra VM Linux e monte esse disco para reparar o problema.
Além disso, para evitar a senha inserida no modo de usuário único, altere da seguinte forma /lib/systemd/system/rescue.service.
--- /lib/systemd/system/rescue.service.orig 2015-11-20 13:49:03.000000000 +0900
+++ /lib/systemd/system/rescue.service 2016-04-11 15:58:31.002000000 +0900
@@ -18,11 +18,11 @@
WorkingDirectory=/root
ExecStartPre=-/bin/plymouth quit
ExecStartPre=-/bin/echo -e 'Welcome to emergency mode! After logging in, type "journalctl -xb" to view\nsystem logs, "systemctl reboot" to reboot, "systemctl default" or ^D to\nboot into default mode.'
-ExecStart=-/bin/sh -c "/usr/sbin/sulogin; /usr/bin/systemctl --fail --no-block default"
+ExecStart=-/bin/bash
Type=idle
StandardInput=tty-force
-StandardOutput=inherit
-StandardError=inherit
+StandardOutput=tty
+StandardError=tty
KillMode=process
IgnoreSIGPIPE=no
SendSIGHUP=yes