xics: add reset() handler to ICPStateClass

xics: add reset() handler to ICPStateClass

Taking into account that qemu_set_irq() returns immediatly if its first
argument is NULL, icp_kvm_reset() largely duplicates icp_reset().

This patch introduces a reset() handler, so that the common logic can
be implemented in icp_reset() only.

While there we can also drop icp_kvm_realize() and icp_kvm_unrealize(). This
causes icp-kvm to be realized in icp_realize(), which sets icp->xics, but
it has no impact.

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