Respond to a query message, return version information

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Implement SR.{attach,detach,create} where device_config contains a single "path"

The "path" will be to a directory, which may or may not exist, which will
contain the virtual disk images and per-disk metadata.

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