spapr: Simplify error handling in spapr_phb_realize()

spapr: Simplify error handling in spapr_phb_realize()

The spapr_phb_realize() function has a local_err variable which
is used to:

1) check failures of spapr_irq_findone() and spapr_irq_claim()

2) prepend extra information to the error message

Recent work from Markus Armbruster highlighted we get better
code when testing the return value of a function, rather than
setting up all the local_err boiler plate. For similar reasons,
it is now preferred to use ERRP_GUARD() and error_prepend()
rather than error_propagate_prepend().

Since spapr_irq_findone() and spapr_irq_claim() return negative
values in case of failure, do both changes.

This is just cleanup, no functional impact.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <159707843851.1489912.6108405733810934642.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>