qapi: Simplify error reporting for array types

check_type() first checks and peels off the array type, then checks
the element type. For two out of four error messages, it takes pains
to report errors for "array of T" instead of just T. Odd. Let's
examine the errors.

* Unknown element type, e.g.
tests/qapi-schema/args-array-unknown.json:

Member 'array' of 'data' for command 'oops' uses unknown type
'array of NoSuchType'

To make sense of this, you need to know that 'array of NoSuchType'
refers to '[NoSuchType]'. Easy enough. However, simply reporting

Member 'array' of 'data' for command 'oops' uses unknown type
'NoSuchType'

is at least as easy to understand.

* Element type's meta-type is inadmissible, e.g.
tests/qapi-schema/returns-whitelist.json:

'returns' for command 'no-way-this-will-get-whitelisted' cannot
use built-in type 'array of int'

'array of int' is technically not a built-in type, but that's
pedantry. However, simply reporting

'returns' for command 'no-way-this-will-get-whitelisted' cannot
use built-in type 'int'

avoids the issue, and is at least as easy to understand.

* The remaining two errors are unreachable, because the array checking
ensures that value is a string.

Thus, reporting some errors for "array of T" instead of just T works,
but doesn't really improve things. Drop it.

Backports commit eddf817bd823a90df209dfbdc2a0b2ec33b7cb77 from qemu

qemu/scripts/qapi.py

@@ -426,7 +426,6 @@ def check_type(expr_info, source, value, allow_array = False,
                allow_dict = False, allow_optional = False,
                allow_star = False, allow_metas = []):
     global all_names
-    orig_value = value
 
     if value is None:
         return
@@ -444,7 +443,6 @@ def check_type(expr_info, source, value, allow_array = False,
                                 "%s: array type must contain single type name"
                                 % source)
         value = value[0]
-        orig_value = "array of %s" %value
 
     # Check if type name for value is okay
     if isinstance(value, str):
@@ -455,11 +453,11 @@ def check_type(expr_info, source, value, allow_array = False,
         if not value in all_names:
             raise QAPIExprError(expr_info,
                                 "%s uses unknown type '%s'"
-                                % (source, orig_value))
+                                % (source, value))
         if not all_names[value] in allow_metas:
             raise QAPIExprError(expr_info,
                                 "%s cannot use %s type '%s'"
-                                % (source, all_names[value], orig_value))
+                                % (source, all_names[value], value))
         return
 
     if not allow_dict: