arm_gicv2m: set kvm_gsi_direct_mapping and kvm_msi_via_irqfd_allowed

After introduction of kvm_arch_msi_data_to_gsi, kvm_gsi_direct_mapping
now can be set on ARM. Also kvm_msi_via_irqfd_allowed can be set,
depending on kernel irqfd support, hence enabling VIRTIO-PCI with
vhost back-end.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>