qapi2texi: replace quotation by bold section name

When we build qemu-qmp-ref.txt this causes texinfo to complain several
times:
"Negative repeat count does nothing at
/usr/share/texinfo/Texinfo/Convert/Line.pm line 124."

It also doesn't display correctly, because the "Notes" text disappears
entirely in the HTML version because it thinks there's no actual
quotation text.

The text file output formatting is also not good.

To solve those problems, remove usage of @quotation, and simply use bold
face for the section name.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170217093416.27688-1-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
