qdev-properties-system: Change set_pointer's parse callback to use Error

qdev-properties-system: Change set_pointer's parse callback to use Error

Instead of having set_pointer() call a parse callback which returns
an error number that we then convert to an Error string with
error_set_from_qdev_prop_error(), make the parse callback take an
Error** and set the error itself. This will allow parse routines
to provide more helpful error messages than the generic ones.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435068107-12594-2-git-send-email-peter.maydell@linaro.org