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 <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20200213175647[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <[email protected]>