When root device is available read-write (TemplateVM/StandaloneVM), its
mounted directly, instead of using device-mapper layer. But
/dev/mapper/dmroot still needs to exists (it is pointed from
/etc/fstab), otherwise various tools, including grub-mkconfig get
confused.
Create a symlink using udev rule. It is already done in initramfs, and
in case of Fedora that udev rule/symlink survive switching to
non-initramfs udev, but not on Debian. So, add appropriate udev rules
file.
FixesQubesOS/qubes-issues#3178
