kvm irqfd: support direct msimessage to irq translation

On PPC64 systems MSI Messages are translated to system IRQ in a PCI
host bridge. This is already supported for emulated MSI/MSIX but
not for irqfd where the current QEMU allocates IRQ numbers from
irqchip and maps MSIMessages to IRQ in the host kernel.

This adds a new direct mapping flag which tells
the kvm_irqchip_add_msi_route() function that a new VIRQ
should not be allocated, instead the value from MSIMessage::data
should be used. It is up to the platform code to make sure that
this contains a valid IRQ number as sPAPR does in spapr_pci.c.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
