Use console=pipe in install_grub() when calling nspawn

Use console=pipe in install_grub() when calling nspawn

The mount bind to /dev on the host conflicts with this patch in systemd,
which doesn't expect /dev/console to exist:
https://github.com/systemd/systemd/commit/3acc84ebd9aebe8cf1771b42644ebbfbecdfaa37

The result is that nspawn dies because it is unable to create a symlink
at /dev/console. This adds a new parameter when calling nspawn to
prevent it from attempting to create/use /dev/console since the default
behavior is to create/use /dev/console and install_grub() doesn't seem to
require an interactive console in order to install grub.

Fixes #380