qapi: leave the ifcond attribute undefined until check()

qapi: leave the ifcond attribute undefined until check()

We commonly initialize attributes to None in .init(), then set their
real value in .check().  Accessing the attribute before .check()
yields None.  If we're lucky, the code that accesses the attribute
prematurely chokes on None.

It won't for .ifcond, because None is a legitimate value.

Leave the ifcond attribute undefined until check().

Suggested-by: Markus Armbruster <[email protected]>
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]>