qapi: Use qemu_strtod_finite() in qobject-input-visitor

Let's use the new function. Just as current behavior, we have to
consume the whole string (now it's just way clearer what's going on).

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181121164421.20780-5-david@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>