test cases/common/215 source set realistic example/devices/meson.build

specific.add(when: 'CONFIG_VIRTIO',    if_true: files('virtio.cc'))
common.add(when: 'CONFIG_VIRTIO_PCI',  if_true: files('virtio-pci.cc'))
common.add(when: 'CONFIG_VIRTIO_MMIO', if_true: files('virtio-mmio.cc'))