dataplane: use linux-headers/ for virtio includes

The hw/dataplane/vring.c code includes linux/virtio_ring.h.  Ensure that
we use linux-headers/ instead of the system-wide headers, which may be
out-of-date on older distros.

This resolves the following build error on Debian 6:

  CC    hw/dataplane/vring.o
cc1: warnings being treated as errors
hw/dataplane/vring.c: In function 'vring_enable_notification':
hw/dataplane/vring.c:71: error: implicit declaration of function 'vring_avail_event'
hw/dataplane/vring.c:71: error: nested extern declaration of 'vring_avail_event'
hw/dataplane/vring.c:71: error: lvalue required as left operand of assignment

Note that we now build dataplane/ for each target instead of only once.
There is no way around this since linux-headers/ is only available for
per-target objects - and it's how virtio, vfio, kvm, and friends are
built.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/Makefile.objs

-common-obj-y = usb/ ide/ pci/ dataplane/
+common-obj-y = usb/ ide/ pci/
 common-obj-y += loader.o
 common-obj-$(CONFIG_VIRTIO) += virtio-console.o
 common-obj-$(CONFIG_VIRTIO) += virtio-rng.o
@@ -191,6 +191,7 @@ common-obj-$(CONFIG_XEN_BACKEND) += xen_console.o xenfb.o xen_disk.o xen_nic.o
 # Per-target files
 # virtio has to be here due to weird dependency between PCI and virtio-net.
 # need to fix this properly
+obj-$(CONFIG_VIRTIO) += dataplane/
 obj-$(CONFIG_VIRTIO) += virtio.o virtio-blk.o virtio-balloon.o virtio-net.o
 obj-$(CONFIG_VIRTIO) += virtio-serial-bus.o virtio-scsi.o
 obj-$(CONFIG_SOFTMMU) += vhost_net.o

hw/dataplane/Makefile.objs

-ifeq ($(CONFIG_VIRTIO), y)
-common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o vring.o event-poll.o ioq.o virtio-blk.o
-endif
+obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o vring.o event-poll.o ioq.o virtio-blk.o