error: preserve errno when saving last error

error: preserve errno when saving last error

It is common to see the sequence:

virErrorPtr save_err = virSaveLastError();
// do cleanup
virSetError(save_err);
virFreeError(save_err);

on cleanup paths.  But for functions where it is desirable to
return the errno that caused failure, this sequence can clobber
that errno.  virFreeError was already safe; this makes the other
two functions in the sequence safe as well, assuming all goes
well (on OOM, errno will be clobbered, but then again, save_err
won't reflect the real error that happened, so you are no longer
preserving the real situation - that's life with OOM).

* src/util/virterror.c (virSaveLastError, virSetError): Preserve
errno.