libxl: Add dmss_init/dispose for libxl__dm_spawn_state

libxl: Add dmss_init/dispose for libxl__dm_spawn_state

These two functions, dmss_init and dmss_dispose, need to be called to
initialise the private parts of a libxl__dm_spawn_state (dmss) as well
as dispose of them before giving back control to a caller.

There are 3 functions that can start using a dmss, the classic
libxl__spawn_local_dm, the one for stubdom libxl__spawn_stub_dm and
libxl__spawn_qdisk_backend. But there are only 2 exit path as
libxl__spawn_qdisk_backend is using libxl__spawn_local_dm functions.

These two new functions are empty but will be used shortly.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>