Bug#904268: debian-installer: Improve instruction when the kernel doesn't support LVM

[root <krichter@posteo.de>]

Package: debian-installer
Severity: normal

Dear Maintainer,

During the installation of a netinst image generated with `jigdo` (`jigdo-lite https://cdimage.debian.org/cdimage/weekly-builds/amd64/jigdo-cd/debian-testing-amd64-netinst.jigdo`) in VirtualBox 5.2.10 I encountered the information message

```
Logical Volume Manager not available
The current kernel doesn't support the Logical Volume Manager. You may need to load the lvm-mod module.
```
in the "Parition disks" section of the graphical installer.

This information is comprehensive, however I'd be much more useful if it would include instructions how to load the kernel module.

Please note that I'm not seeking support is this matter, my intention is to make the installation more intuitive.


-- System Information:
Debian Release: buster/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.15.0-24-generic (SMP w/8 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968), LANGUAGE=C (charmap=ANSI_X3.4-1968)
Shell: /bin/sh linked to /usr/bin/dash
Init: unable to detect