fix detection of Linux MD for grub2 (bnc#811830)

Add code to detect installation on Linux MD RAID. perl-Bootloader
already was able to install on multiple devices, but it always got
single device from YaST2.

This is expected to be safe because grub2-install requires embedding
support in case /boot/grub2 is on RAID device.

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>