libxl_disk: Cut libxl_cdrom_insert into steps ..

libxl_disk: Cut libxl_cdrom_insert into steps ..

.. and use a new "slow" lock to avoid holding the userdata lock across
several functions.

This patch cuts libxl_cdrom_insert into different step/function but
there are still called synchronously. (Taking the ev_lock is the only
step that might be asynchronous.) A later patch will call them
asynchronously when QMP is involved.

Thee userdata lock (json_lock) use to protect against concurrent change
of cdrom is replaced by an ev_lock which can be held across different
CTX_LOCK sections. The json_lock is still used when reading/modifying
the domain userdata (mandatory) and update xenstore (mostly because
it's updated as the same time as the userdata).

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