Correct the calculating of disk size when using lvm disk backend.

Currently, disk space size calculating in nova doesn't consider the
situation when lvm is used as disk backend. Thus both used and total
disk size are not correct when we use lvm to create instance. This
patch fixes this problem by using 'vgs' to get disk size in such case.

Fixes LP #1098116.

Change-Id: I0e86649cfde83a154d0a4c034ca52abe73384e73
Merge "Correct the calculating of disk size when using lvm disk backend."