snapshot_disk: Add pool volume snapshot cases

Current test is with snapshot disk in a created tmp dir, for support
pool volume testing with snapshot, create certain type pool and use
volume in the pool for snapshot testing.

Skip specific cases since upstream and rhel7 change.

Covered pool: "dir", "fs", "netfs", "iscsi", "logical" and "disk"

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #212 from waynesun09/snapshot_disk_with_pool

snapshot_disk: Add pool volume snapshot cases