xend: NetBSD portability fix for LVM raw volume names

xend: NetBSD portability fix for LVM raw volume names

Xen 4.1.1 was incorrectly passing /dev/rmapper/vg-lvname to pygrub
(notice the r in front of mapper), when it should pass
/dev/mapper/rvg-lvname (add the r to the last file) when using NetBSD.
I've patched it to work correctly. I'm attaching a unified diff with
the patch made against Xen 4.1.1 (it's a really simple modification).

From: Roger Pau Monne <roger.pau@entel.upc.edu>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>