xics-kvm: enable irqfd for MSI

xics-kvm: enable irqfd for MSI

This enables IRQFD support for sPAPR. The feature decreases the latency
of interrupt handling.

To enable IRQFD for MSI, this sets kvm_gsi_direct_mapping to true which
enables direct MSI mapping.

To enable IRQFD for LSI (level triggered INTx interrupts), a PCI host bus
callback is required. The patch for that is coming next.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>