block: Use new AsyncContext for bdrv_read/write emulation

bdrv_read/write emulation is used as the perfect example why we need something
like AsyncContexts. So maybe they better start using it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block.c

@@ -1696,19 +1696,26 @@ static int bdrv_read_em(BlockDriverState *bs, int64_t sector_num,
     struct iovec iov;
     QEMUIOVector qiov;
 
+    async_context_push();
+
     async_ret = NOT_DONE;
     iov.iov_base = (void *)buf;
     iov.iov_len = nb_sectors * 512;
     qemu_iovec_init_external(&qiov, &iov, 1);
     acb = bdrv_aio_readv(bs, sector_num, &qiov, nb_sectors,
         bdrv_rw_em_cb, &async_ret);
-    if (acb == NULL)
-        return -1;
+    if (acb == NULL) {
+        async_ret = -1;
+        goto fail;
+    }
 
     while (async_ret == NOT_DONE) {
         qemu_aio_wait();
     }
 
+
+fail:
+    async_context_pop();
     return async_ret;
 }
 
@@ -1720,17 +1727,24 @@ static int bdrv_write_em(BlockDriverState *bs, int64_t sector_num,
     struct iovec iov;
     QEMUIOVector qiov;
 
+    async_context_push();
+
     async_ret = NOT_DONE;
     iov.iov_base = (void *)buf;
     iov.iov_len = nb_sectors * 512;
     qemu_iovec_init_external(&qiov, &iov, 1);
     acb = bdrv_aio_writev(bs, sector_num, &qiov, nb_sectors,
         bdrv_rw_em_cb, &async_ret);
-    if (acb == NULL)
-        return -1;
+    if (acb == NULL) {
+        async_ret = -1;
+        goto fail;
+    }
     while (async_ret == NOT_DONE) {
         qemu_aio_wait();
     }
+
+fail:
+    async_context_pop();
     return async_ret;
 }