qapi/block-core.json: Use literal block for ascii art

The ascii-art graph in the BlockLatencyHistogramInfo documentation
doesn't render correctly, because the whitespace is collapsed.

Use the '|' format that emits a literal 'example' block so the graph
is displayed correctly.

Strictly the Texinfo generated is still wrong because each line
goes into its own @example environment, but it renders better
than what we had before.

Fixing this rendering is a necessary prerequisite for the upcoming rST
generator, which otherwise complains about the inconsistent
indentation in the ascii-art graph.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200213175647.17628-8-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/block-core.json

@@ -550,13 +550,13 @@
 #        For the example above, @bins may be something like [3, 1, 5, 2],
 #        and corresponding histogram looks like:
 #
-#        5|           *
-#        4|           *
-#        3| *         *
-#        2| *         *    *
-#        1| *    *    *    *
-#         +------------------
-#             10   50   100
+# |      5|           *
+# |      4|           *
+# |      3| *         *
+# |      2| *         *    *
+# |      1| *    *    *    *
+# |       +------------------
+# |           10   50   100
 #
 # Since: 4.0
 ##