qapi: Emit structs used as variants in topological order

qapi: Emit structs used as variants in topological order

Right now, we emit the branches of union types as a boxed pointer,
and it suffices to have a forward declaration of the type.  However,
a future patch will swap things to directly use the branch type,
instead of hiding it behind a pointer.  For this to work, the
compiler needs the full definition of the type, not just a forward
declaration, prior to the union that is including the branch type.
This patch just adds topological sorting to hoist all types
mentioned in a branch of a union to be fully declared before the
union itself.  The sort is always possible, because we do not
allow circular union types that include themselves as a direct
branch (it is, however, still possible to include a branch type
that itself has a pointer to the union, for a type that can
indirectly recursively nest itself - that remains safe, because
that the member of the branch type will remain a pointer, and the
QMP representation of such a type adds another {} for each recurring
layer of the union type).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-11-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>