KVM: PPC: Book3S HV: XIVE: Add a control to configure a source

This control will be used by the H_INT_SET_SOURCE_CONFIG hcall from
QEMU to configure the target of a source and also to restore the
configuration of a source when migrating the VM.

The XIVE source interrupt structure is extended with the value of the
Effective Interrupt Source Number. The EISN is the interrupt number
pushed in the event queue that the guest OS will use to dispatch
events internally. Caching the EISN value in KVM eases the test when
checking if a reconfiguration is indeed needed.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>