virtio-9p: fix compilation error.

Fix the compilation error introduced by msg new field.

CC    hw/9pfs/virtio-9p.o
In file included from /home/konradf/Documents/safe/greensocs/virtio-project/x86-qemu/qemu/hw/9pfs/virtio-9p.c:17:0:
/home/konradf/Documents/safe/greensocs/virtio-project/x86-qemu/qemu/hw/virtio-pci.h:30:16: erreur: field ‘msg’ has incomplete type
make: *** [hw/9pfs/virtio-9p.o] Erreur 1

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>