bug#62140: enable LVM in Grub

[Emmanuel Beffara]

Hello Guix,

Unless I missed something in the configuration process, the `grub.cfg`
generated by `guix system reconfigure` does not enable LVM, even if the store
is in an LVM volume. The immediate consequence is that Grub cannot load its
data files nor boot the system. The fix is as simple as adding `insmod lvm` in
`grub.cfg` before any `search` command, which I do by hand on each
`reconfigure`, but obviously Guix itself should do that.

I looked at `grub.scm` to see where things happen, and I see no mention of LVM
in this file. This surprises me because I am probably not the only one using
it. Actually, I use LVM on LUKS for full-system encryption, but this is
probably not rare either.

-- 
Emmanuel