"sys" handler tries to obtain partition table from DRBD devices and fails.

When using a "sys"-type handler with the "partitions" option set to yes, on hosts where some DRBD devices are setup, I'm getting an error on every run:

== warnings from /etc/backup.d/30_main.sys ==

Warning: The partition table for /dev/drbd1 could not be saved.
Warning: The partition table for /dev/drbd1 could not be saved.
Warning: The partition table for /dev/drbd1 could not be saved.
Warning: The partition table for /dev/drbd1 could not be saved.
Warning: The partition table for /dev/drbd2 could not be saved.
Warning: The partition table for /dev/drbd2 could not be saved.
Warning: The partition table for /dev/drbd2 could not be saved.
Warning: The partition table for /dev/drbd2 could not be saved.

From a quick reading of the sys handler, a fix for this could be to simply "grep -v ^/dev/drbd" like it is done for zram.