[FIX] Get physical device name

/^\wd\w+$/ get unnecessary partition name such as sda1 or sdb2.
/^[a-z]d[a-z]+$/ exclude figures of partitions.

see also https://github.com/tacahilo/munin-plugins/pull/3
Merge pull request #3 from tacahilo/fix-partition-list

[FIX] Get physical device name