spapr, xics, xive: Match signatures for XICS and XIVE KVM connect routines

spapr, xics, xive: Match signatures for XICS and XIVE KVM connect routines

Both XICS and XIVE have routines to connect and disconnect KVM with
similar but not identical signatures.  This adjusts them to match
exactly, which will be useful for further cleanups later.

While we're there, we add an explicit return value to the connect path
to streamline error reporting in the callers.  We remove error
reporting the disconnect path.  In the XICS case this wasn't used at
all.  In the XIVE case the only error case was if the KVM device was
set up, but KVM didn't have the capability to do so which is pretty
obviously impossible.

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