qapi: Add blank lines before bulleted lists

qapi: Add blank lines before bulleted lists

We would like to switch the doc comments to rST format. rST
insists on a blank line before and after a bulleted list, but our
Texinfo doc generator did not. Add some extra blank lines in the doc
comments so they're acceptable rST input.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200213175647.17628-17-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>