tools/libxl: Make libxl__conversion_helper_abort() safe to use

tools/libxl: Make libxl__conversion_helper_abort() safe to use

Previously, in the case of an error causing a call to
libxl__conversion_helper_abort() on a stream without legacy conversion,
libxl would fall over a NULL pointer because chs->ao was not set up.

Arrange for all ->ao's to be set up at _init() time, by having each
_init() function assert that their caller has done the right thing.
While doing so, introduce a previously-missing save_helper_init() in
stream_read_init().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>