Fix undefined method: inject for String

The physical_volumes extraction using inject function doesn't work.
Change to using split/grep/map.

/Stage[main]/Releng::Disks/Volume_group[vg] (err): Could not evaluate: undefined method `inject' for "  PV,VG\n/dev/sda4,vg\n":String

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>