virtio: move VirtQueueElement at the beginning of the structs

The next patch will make virtqueue_pop/vring_pop allocate memory for
the VirtQueueElement. In some cases (blk, scsi, gpu) the device wants
to extend VirtQueueElement with device-specific fields and, until now,
the place of the VirtQueueElement within the containing struct didn't
matter. When allocating the entire block in virtqueue_pop/vring_pop,
however, the containing struct must basically be a "subclass" of
VirtQueueElement, with the VirtQueueElement as the first field. Make
that the case for blk and scsi; gpu is already doing it.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/scsi/virtio-scsi.c

@@ -45,8 +45,7 @@ VirtIOSCSIReq *virtio_scsi_init_req(VirtIOSCSI *s, VirtQueue *vq)
 {
     VirtIOSCSIReq *req;
     VirtIOSCSICommon *vs = (VirtIOSCSICommon *)s;
-    const size_t zero_skip = offsetof(VirtIOSCSIReq, elem)
-                             + sizeof(VirtQueueElement);
+    const size_t zero_skip = offsetof(VirtIOSCSIReq, vring);
 
     req = g_malloc(sizeof(*req) + vs->cdb_size);
     req->vq = vq;

include/hw/virtio/virtio-blk.h

@@ -60,9 +60,9 @@ typedef struct VirtIOBlock {
 } VirtIOBlock;
 
 typedef struct VirtIOBlockReq {
+    VirtQueueElement elem;
     int64_t sector_num;
     VirtIOBlock *dev;
-    VirtQueueElement elem;
     struct virtio_blk_inhdr *in;
     struct virtio_blk_outhdr out;
     QEMUIOVector qiov;

include/hw/virtio/virtio-scsi.h

@@ -112,18 +112,17 @@ typedef struct VirtIOSCSI {
 } VirtIOSCSI;
 
 typedef struct VirtIOSCSIReq {
+    /* Note:
+     * - fields up to resp_iov are initialized by virtio_scsi_init_req;
+     * - fields starting at vring are zeroed by virtio_scsi_init_req.
+     * */
+    VirtQueueElement elem;
+
     VirtIOSCSI *dev;
     VirtQueue *vq;
     QEMUSGList qsgl;
     QEMUIOVector resp_iov;
 
-    /* Note:
-     * - fields before elem are initialized by virtio_scsi_init_req;
-     * - elem is uninitialized at the time of allocation.
-     * - fields after elem are zeroed by virtio_scsi_init_req.
-     * */
-
-    VirtQueueElement elem;
     /* Set by dataplane code. */
     VirtIOSCSIVring *vring;