win32: Fix compiler errors from u_int64_t

win32: Fix compiler errors from u_int64_t

u_int64_t raises compiler error messages:

  CC    libhw32/virtio.o
/qemu/ar7/hw/virtio.c: In function ‘virtio_queue_get_avail_size’:
/qemu/ar7/hw/virtio.c:776: error: ‘u_int64_t’ undeclared (first use in this function)
/qemu/ar7/hw/virtio.c:776: error: (Each undeclared identifier is reported only once
/qemu/ar7/hw/virtio.c:776: error: for each function it appears in.)

Replacing u_int64_t by uint64_t helps.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>