spapr: Improve spapr_reallocate_hpt() error reporting

spapr_reallocate_hpt() has three users, two of which pass &error_fatal
and the third one, htab_load(), passes &local_err, uses it to detect
failures and simply propagates -EINVAL up to vmstate_load(), which will
cause QEMU to exit. It is thus confusing that spapr_reallocate_hpt()
doesn't return right away when an error is detected in some cases. Also,
the comment suggesting that the caller is welcome to try to carry on
seems like a remnant in this respect.

This can be improved:
- change spapr_reallocate_hpt() to always report a negative errno on
  failure, either as reported by KVM or -ENOSPC if the HPT is smaller
  than what was asked,
- use that to detect failures in htab_load() which is preferred over
  checking &local_err,
- propagate this negative errno to vmstate_load() because it is more
  accurate than propagating -EINVAL for all possible errors.

[dwg: Fix compile error due to omitted prelim patch]
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160371605460.305923.5890143959901241157.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>