qapi: Add alias for ErrorClass

The qapi enum ErrorClass is unusual that it uses 'CamelCase' names,
contrary to our documented convention of preferring 'lower-case'.
However, this enum is entrenched in the API; we cannot change
what strings QMP outputs. Meanwhile, we want to simplify how
c_enum_const() is used to generate enum constants, by moving away
from the heuristics of camel_to_upper() to a more straightforward
c_name(N).upper() - but doing so will rename all of the ErrorClass
constants and cause churn to all client files, where the new names
are aesthetically less pleasing (ERROR_CLASS_DEVICENOTFOUND looks
like we can't make up our minds on whether to break between words).

So as always in computer science, solve the problem by some more
indirection: rename the qapi type to QapiErrorClass, and add a
new enum ErrorClass in error.h whose members are aliases of the
qapi type, but with the spelling expected elsewhere in the tree.
Then, when c_enum_const() changes the munging, we only have to
adjust the one alias spot.

Backports commit f22a28b898322c01b0463a8b7ec551d72bc61a5b from qemu

qemu/include/qapi/error.h

@@ -111,6 +111,20 @@
  */
 typedef struct Error Error;
 
+/*
+ * Overall category of an error.
+ * Based on the qapi type QapiErrorClass, but reproduced here for nicer
+ * enum names.
+ */
+typedef enum ErrorClass {
+    ERROR_CLASS_GENERIC_ERROR = QAPI_ERROR_CLASS_GENERIC_ERROR,
+    ERROR_CLASS_COMMAND_NOT_FOUND = QAPI_ERROR_CLASS_COMMAND_NOT_FOUND,
+    ERROR_CLASS_DEVICE_ENCRYPTED = QAPI_ERROR_CLASS_DEVICE_ENCRYPTED,
+    ERROR_CLASS_DEVICE_NOT_ACTIVE = QAPI_ERROR_CLASS_DEVICE_NOT_ACTIVE,
+    ERROR_CLASS_DEVICE_NOT_FOUND = QAPI_ERROR_CLASS_DEVICE_NOT_FOUND,
+    ERROR_CLASS_KVM_MISSING_CAP = QAPI_ERROR_CLASS_KVM_MISSING_CAP,
+} ErrorClass;
+
 /**
  * Get @err's human-readable error message.
  */

qemu/qapi/common.json

@@ -3,7 +3,7 @@
 # QAPI common definitions
 
 ##
-# @ErrorClass
+# @QapiErrorClass
 #
 # QEMU error classes
 #
@@ -24,7 +24,8 @@
 #
 # Since: 1.2
 ##
-{ 'enum': 'ErrorClass',
+{ 'enum': 'QapiErrorClass',
+  # Keep this in sync with ErrorClass in error.h
   'data': [ 'GenericError', 'CommandNotFound', 'DeviceEncrypted',
             'DeviceNotActive', 'DeviceNotFound', 'KVMMissingCap' ] }