Comment 19 for bug 949992

You should run 'sudo dpkg-reconfigure grub-pc' and instruct it to install the boot loader somewhere, probably /dev/vda.

The debconf database shows that the following question was presented to you at some point:

  You chose not to install GRUB to any devices. If you continue, the boot loader may not be properly configured, and when this computer next starts up it will use whatever was previously in the boot sector. If there is an earlier version of GRUB 2 in the boot sector, it may be unable to load modules or handle the current configuration file.

  If you are already using a different boot loader and want to carry on doing so, or if this is a special environment where you do not need a boot loader, then you should continue anyway. Otherwise, you should install GRUB somewhere.

  Continue without installing GRUB?

Do you remember seeing this question? It was intended to strongly discourage setups like this ...