extrahd.cgi: Add support for LVM and MDADM devices

extrahd.cgi: Add support for LVM and MDADM devices

This commit adds support for using LVM and mdadm based RAID devices
for the CGI page.

In case one or more drives/partitions are used by such a "grouped"
volume they still will displayed on the page, but can not be
configured/used. Instead the "master" volume of which the
drive/partition is part of is shown in the "mountpoint" input box.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>