spapr: check for the activation of the KVM IRQ device

The activation of the KVM IRQ device depends on the interrupt mode
chosen at CAS time by the machine and some methods used at reset or by
the migration need to be protected.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190513084245.25755-11-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>