sd-bus: fix encoding/decoding gvariant root container

The gvariant root container contains a 'variant' at the end, which embeds
the whole message body. This variant *must* contain a structure so we are
compatible to dbus1. Otherwise, it could encode at most 1 type, instead
of a full signature.

Our gvariant message parser already parses the variant-content as a
structure, so we're mostly good. However, it does *not* include the
opening and closing parantheses, nor does it parse them.

This patch fixes the decoder to verify a message contains the
parantheses, and also make the encoder add those parantheses into the
marshaled message.