msg: add factory method to correctly construct smart_ptr

msg: add factory method to correctly construct smart_ptr

This is to avoid easy leaks of the form:

    boost::intrusive_ptr(new M*(...));

where a ref is leaked because constructing the intrusive_ptr adds a ref.

It is expected that eventually all constructors move to protected/private so
that the message's factory is the only way to build a new message (safely).

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>