qapi: add 'if' to alternate members

qapi: add 'if' to alternate members

Add 'if' key to alternate members:

{ 'alternate': 'TestIfAlternate', 'data':
  { 'alt': { 'type': 'TestStruct', 'if': 'COND' } } }

Generated code is not changed by this patch but with "qapi: add #if
conditions to generated code".

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-17-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>