qapi: fix examples of blockdev-add with qcow2

The examples use "qcow2" driver with the wrong member name for
BlockdevRef alternate type. This patch changes all wrong member names
from "file" to "data-file" which is the correct member name in
BlockdevOptionsQcow2 for the BlockdevRef field.

Problem was noticed when using the example as a test case for Go
bindings.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Message-Id: <20220901085840.22520-9-victortoso@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/block-core.json

@@ -1541,8 +1541,8 @@
 # -> { "execute": "blockdev-add",
 #      "arguments": { "driver": "qcow2",
 #                     "node-name": "node1534",
-#                     "file": { "driver": "file",
-#                               "filename": "hd1.qcow2" },
+#                     "data-file": { "driver": "file",
+#                                    "filename": "hd1.qcow2" },
 #                     "backing": null } }
 #
 # <- { "return": {} }
@@ -4378,7 +4378,7 @@
 #      "arguments": {
 #           "driver": "qcow2",
 #           "node-name": "test1",
-#           "file": {
+#           "data-file": {
 #               "driver": "file",
 #               "filename": "test.qcow2"
 #            }
@@ -4395,7 +4395,7 @@
 #           "cache": {
 #              "direct": true
 #            },
-#            "file": {
+#           "data-file": {
 #              "driver": "file",
 #              "filename": "/tmp/test.qcow2"
 #            },
@@ -4477,7 +4477,7 @@
 #      "arguments": {
 #           "driver": "qcow2",
 #           "node-name": "node0",
-#           "file": {
+#           "data-file": {
 #               "driver": "file",
 #               "filename": "test.qcow2"
 #           }