Fix error casting value to float in lvm.py

Special locale setting may result in that "vgs" command output different
results. Such as:

$ LC_NUMERIC='es_ES.UTF-8' sudo vgs
VG #PV #LV #SN Attr VSize VFree
vg_eharney 1 6 1 wz--n- 465,25g 12,97g

Fix bug 1219963

Change-Id: Ibe79d5ce673738d8e37f86de9106ab347c21bd52
Merge "Fix error casting value to float in lvm.py"