Allow the SMAPIv2 default socket to be overriden, same as the xenops one.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add a storage skeleton implementation which raises Unimplemented for most methods

Simple storage backends can 'include' the skeleton and selectively
override modules.

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