Log every subcommand invocation

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Replace strings with the Common.format variant

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Snapshot and clone should inherit virtual_size from the parent

It's possible to create leaves with different sizes, but we don't
want to do that in the SMAPI VDI.{clone,snapshot} APIs.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Improve the README.md

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.15

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #15 from djs55/add-qcow2

Fix .qcow2 snapshot and clone