virtio-blk: fix reference a pointer which might be freed

virtio-blk: fix reference a pointer which might be freed

In function virtio_blk_handle_request, it may freed memory pointed by req,
So do not access member of req after calling this function.

Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 1bdb176ac5add5dc9d54a230da7511b66851f1e7)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>