sheepdog: fix NULL dereference in sd_create

Following command

qemu-img create -f qcow2 sheepdog:test 20g

will cause core dump because aio_context is NULL in sd_create. We should
initialize it by qemu_get_aio_context() to avoid NULL dereference.

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/sheepdog.c

@@ -1756,6 +1756,7 @@ static int sd_create(const char *filename, QemuOpts *opts,
         bdrv_unref(bs);
     }
 
+    s->aio_context = qemu_get_aio_context();
     ret = do_sd_create(s, &vid, 0, errp);
     if (ret) {
         goto out;