qapi: Rewrite parsing of doc comment section symbols and tags

qapi: Rewrite parsing of doc comment section symbols and tags

To recognize a line starting with a section symbol and or tag, we
first split it at the first space, then examine the part left of the
space.  We can just as well examine the unsplit line, so do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-13-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[Work around lack of walrus operator in Python 3.7 and older]