ppc/kvm: change kvmppc_get_htab_fd() to return -errno on error

When kvmppc_get_htab_fd() fails, its return value is propagated up to
qemu_savevm_state_iterate() or to qemu_savevm_state_complete_precopy().
All savevm handlers expect to receive a negative errno on error.

Let's patch kvmppc_get_htab_fd() accordingly.

While here, let's change htab_load() in the spapr code to also
propagate the error, since it doesn't make sense to abort() if
we couldn't get the htab fd from KVM.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>