qapi: replace if condition list with dict {'all': [...]}

Replace the simple list sugar form with a recursive structure that will
accept other operators in the following commits (all, any or not).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210804083105.97531-7-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Accidental code motion undone.  Degenerate :forms: comment dropped.
Helper _check_if() moved.  Error messages tweaked.  ui.json updated.
Accidental changes to qapi-schema-test.json dropped.]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qapi-schema/doc-good.out

@@ -12,15 +12,16 @@ enum QType
 module doc-good.json
 enum Enum
     member one
-        if ['defined(IFONE)']
+        if defined(IFONE)
     member two
-    if ['defined(IFCOND)']
+    if defined(IFCOND)
     feature enum-feat
 object Base
     member base1: Enum optional=False
+    if OrderedDict([('all', ['IFALL1', 'IFALL2'])])
 object Variant1
     member var1: str optional=False
-        if ['defined(IFSTR)']
+        if defined(IFSTR)
         feature member-feat
     feature variant1-feat
 object Variant2
@@ -29,7 +30,7 @@ object Object
     tag base1
     case one: Variant1
     case two: Variant2
-        if ['IFTWO']
+        if IFTWO
     feature union-feat1
 object q_obj_Variant1-wrapper
     member data: Variant1 optional=False
@@ -38,13 +39,13 @@ object q_obj_Variant2-wrapper
 enum SugaredUnionKind
     member one
     member two
-        if ['IFTWO']
+        if IFTWO
 object SugaredUnion
     member type: SugaredUnionKind optional=False
     tag type
     case one: q_obj_Variant1-wrapper
     case two: q_obj_Variant2-wrapper
-        if ['IFTWO']
+        if IFTWO
     feature union-feat2
 alternate Alternate
     tag type