Chances are your missing the udev rules needed to create the /dev/root symlink. Create /etc/udev/rules.d/99-dev-root.rules and add the following into it. Once that's done your server will create the /dev/root symlink on boot.

KERNEL=="xvda", SUBSYSTEM=="block", SYMLINK+="root"

If you do not want to reboot after doing the above you can also do the following which will create the symlink by hand. You still must do the above or the symlink will disappear on reboots

ln -s /dev/xvda /dev/root