qapi: Improve how keyval input visitor reports unexpected dicts

Incorrect option

    -blockdev node-name=foo,driver=file,filename=foo.img,aio.unmap=on

is rejected with "Invalid parameter type for 'aio', expected: string".
To make sense of this, you almost have to translate it into the
equivalent QMP command

    { "execute": "blockdev-add", "arguments": { "node-name": "foo", "driver": "file", "filename": "foo.img", "aio": { "unmap": true } } }

Improve the error message to "Parameters 'aio.*' are unexpected".
Take care not to confuse the case "unexpected nested parameters"
(i.e. the object is a QDict or QList) with the case "non-string scalar
parameter".  The latter is a misuse of the visitor, and should perhaps
be an assertion.  Note that test-qobject-input-visitor exercises this
misuse in test_visitor_in_int_keyval(), test_visitor_in_bool_keyval()
and test_visitor_in_number_keyval().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1488317230-26248-23-git-send-email-armbru@redhat.com>
