spapr/pci: Consolidate de-allocation of MSIs

spapr/pci: Consolidate de-allocation of MSIs

When freeing MSIs, we need to:
- remove them from the machine's MSI bitmap
- remove them from the IC backend
- remove them from the PHB's MSI cache

This is currently open coded in two places in rtas_ibm_change_msi(),
and we're about to need this in spapr_phb_reset() as well. Instead of
duplicating this code again, make it a destroy function for the PHB's
MSI cache. Removing an MSI device from the cache will call the destroy
function internally.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <156415227855.1064338.5657793835271464648.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>