qapi: pass 'if' condition into QAPISchemaEntity objects

qapi: pass 'if' condition into QAPISchemaEntity objects

Built-in objects remain unconditional.  Explicitly defined objects use
the condition specified in the schema.  Implicitly defined objects
inherit their condition from their users.  For most of them, there is
exactly one user, so the condition to use is obvious.  The exception
is wrapped types generated for simple union variants, which can be
shared by any number of simple unions.  The tight condition would be
the disjunction of the conditions of these simple unions.  For now,
use the wrapped type's condition instead.  Much simpler and good
enough for now.

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Message-Id: <20180703155648[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>