Replaces call to lvs with blockdev.

Replaces the call to lvs in virt/libvirt/utils.py:logical_volume_size()
with the more general blockdev.

Implements: blueprint encrypt-ephemeral-storage
Change-Id: I3e4e4e927a98fa9e396618e45ac01cc836a75988
Merge "Replaces call to lvs with blockdev."