libxl: adjust handling of libxl_device_disk objects

libxl: adjust handling of libxl_device_disk objects

libxl objects are supposed to be initialized and disposed.
Correct the usage of libxl_device_disk objects which are allocated on
the stack. Initialize each one prior usage, and dispose them once done.

Adjust libxlMakeDisk to use an already initialized object, it is owned
by the caller.

Adjust libxlMakeDiskList to initialize the list of objects, before they
are filled by libxlMakeDisk. In case of error, the objects are disposed
by libxl_domain_config_dispose.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>