xive: Improve irq claim/free path

xive: Improve irq claim/free path

spapr_xive_irq_claim() returns a bool to indicate if it succeeded.
But most of the callers and one callee use int return values and/or an
Error * with more information instead.  In any case, ints are a more
common idiom for success/failure states than bools (one never knows
what sense they'll be in).

So instead change to an int return value to indicate presence of error
+ an Error * to describe the details through that call chain.

It also didn't actually check if the irq was already claimed, which is
one of the primary purposes of the claim path, so do that.

spapr_xive_irq_free() also returned a bool... which no callers checked
and was always true, so just drop it.

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