Allow equal sign and coma in feature requests

It is necessary for boot mode feature, as a common characters in
kernelopts.

QubesOS/qubes-issues#9750

qubes/api/misc.py

@@ -58,7 +58,7 @@ async def qubes_features_request(self):
             for key in keys
         }
 
-        safe_set = string.ascii_letters + string.digits + "-._ "
+        safe_set = string.ascii_letters + string.digits + "-.,_= "
         for untrusted_key in untrusted_features:
             untrusted_value = untrusted_features[untrusted_key]
             self.enforce(all((c in safe_set) for c in untrusted_value))