qapi: Reject multiple and empty feature descriptions

qapi: Reject multiple and empty feature descriptions

The parser recognizes only the first "Features:" line.  Any subsequent
ones are treated as ordinary text, as visible in test case
doc-duplicate-features.  Recognize "Features:" lines anywhere.  A
second one is an error.

A 'Features:' line without any features is useless, but not an error.
Make it an error.  This makes detecting a second "Features:" line
easier.

qapi/run-state.json actually has an instance of this since commit
fe17522d854 (qapi: Remove deprecated 'singlestep' member of
StatusInfo).  Clean it up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-16-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>