libxl: Fix uninitialised rc in libxl__domain_save_device_model

libxl: Fix uninitialised rc in libxl__domain_save_device_model

c3c8da9 "libxl: ao: datacopier callback gets an rc" caused
libxl__domain_save_device_model() to pass its rc directly into the
callback.

However in the preexisting code, there were 3 "goto out;" paths which
left rc uninitialised.  This causes a build failure with GCC 4.8's
-Wmaybe-uninitialized.

Set the rc explicitly on each goto out path.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>