qapi: Check feature documentation against the schema

Commit f3ed93d545 "qapi: Allow documentation for features" neglected
to check documentation against the schema.  Fix that: check them the
same way we check arguments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-20-armbru@redhat.com>

tests/qapi-schema/doc-bad-feature.err

@@ -0,0 +1 @@
+doc-bad-feature.json:3: documented member 'a' does not exist

tests/qapi-schema/doc-bad-feature.json

@@ -1,5 +1,4 @@
 # Features listed in the doc comment must exist in the actual schema
-# BUG: nonexistent @a is not rejected
 
 ##
 # @foo:

tests/qapi-schema/doc-bad-feature.out

@@ -1,19 +0,0 @@
-module None
-object q_empty
-enum QType
-    prefix QTYPE
-    member none
-    member qnull
-    member qnum
-    member qstring
-    member qdict
-    member qlist
-    member qbool
-module doc-bad-feature.json
-command foo None -> None
-    gen=True success_response=True boxed=False oob=False preconfig=False
-doc symbol=foo
-    body=
-
-    feature=a
-a

tests/qapi-schema/doc-undoc-feature.err

@@ -0,0 +1,2 @@
+doc-undoc-feature.json: In command 'foo':
+doc-undoc-feature.json:9: feature 'undoc' lacks documentation

tests/qapi-schema/doc-undoc-feature.json

@@ -1,5 +1,4 @@
 # Doc comment must cover all features
-# BUG: missing documentation for @undoc not caught
 
 ##
 # @foo:

tests/qapi-schema/doc-undoc-feature.out

@@ -1,21 +0,0 @@
-module None
-object q_empty
-enum QType
-    prefix QTYPE
-    member none
-    member qnull
-    member qnum
-    member qstring
-    member qdict
-    member qlist
-    member qbool
-module doc-undoc-feature.json
-command foo None -> None
-    gen=True success_response=True boxed=False oob=False preconfig=False
-    feature undoc
-    feature doc
-doc symbol=foo
-    body=
-
-    feature=doc
-documented feature