VDI.{clone,snapshot}: ensure we set sm_config keys properly

The backend uses the sm_config keys for its own purposes. In our
case we use them to track the format of the disk. We must use
our values for the sm_config in snapshot/clone, not any values
sent to us by the client.

Fixes #12

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