block.py:  Changes to support thin provisioning.

Users can now use the syntax of vg/lv where the lv refers to a thinly
provisioned pool which can be used to create logical volumes from.

Otherwise the functionality will remain as it previously was.

Copies will utilize thinp snapshots for thinp pools, resulting in very
quick replication and space savings.

Signed-off-by: Tony Asleson <tasleson@redhat.com>