Create lvm.py module containing helper API for LVM

Currently there are some LVM API calls in 'nova/virt/libvirt/utils.py'
and also many other APIs are there.
To make a role of the code be clear, I would like to suggest to
create 'nova/virt/libvirt/lvm.py' and move APIs which are related
to LVM to lvm.py.

Change-Id: I4d80f574631d52012152c72d1db3069dd57a5550
Signed-off-by: Mitsuhiro Tanino <mitsuhiro.tanino@hds.com>
Implements: blueprint lvm-driver-for-shared-storage
Merge "Create lvm.py module containing helper API for LVM"