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>