qapi: Normalize 'if' in check_exprs(), like other sugar

qapi: Normalize 'if' in check_exprs(), like other sugar

We normalize shorthand to longhand forms in check_expr(): enumeration
values with normalize_enum(), feature values with
normalize_features(), struct members, union branches and alternate
branches with normalize_members().  If conditions are an exception: we
normalize them in QAPISchemaEntity.check() and
QAPISchemaMember.__init(), with listify_cond().  The idea goes back to
commit 2cbc94376e "qapi: pass 'if' condition into QAPISchemaEntity
objects", v3.0.0.

Normalize in check_expr() instead, with new helper normalize_if().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-14-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>