qapi/introspect.py: use _make_tree for features nodes

qapi/introspect.py: use _make_tree for features nodes

At present, we open-code this in _make_tree itself; but if the structure
of the tree changes, this is brittle. Use an explicit recursive call to
_make_tree when appropriate to help keep the interior node typing
consistent.

A consequence of doing this is that the 'ifcond' key of the features
dict will be omitted when ifcond is false-ish, just like it is omitted
in top-level calls to _make_tree. This also increases consistency in our
handling of this property.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>