qom: centralize module-loading functionality

qom: centralize module-loading functionality

Put together the common code of object_initialize() and
module_object_class_by_name() into a function that supports
Error **.  Rename the existing function type_get_by_name() to
clarify that it will only look at defined types; this is often
okay within object.c to look at the parents, but not outside it.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>