xen: Use auto keyword as per C23

In macros it is common to declare local variables using typeof(param) in order
to ensure that side effects are only evaluated once.  A consequence of this is
double textural expansion of the parameter, which can get out of hand very
quickly with nested macros.

In C23, the auto keyword has been repurposed to perform type inference.

A GCC extension, __auto_type, is now available in the new toolchain baseline
and avoids the double textural expansion.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Frediano Ziglio <frediano.ziglio@cloud.com>

CODING_STYLE

@@ -129,6 +129,11 @@ Fixed width types should only be used when a fixed width quantity is
 meant (which for example may be a value read from or to be written to a
 register).
 
+Macros which otherwise would use "typeof(arg) newarg =" to avoid double
+evaluation of side effects should use "auto newarg =" per it's C23 behaviour,
+to also avoid double textural expansion.  The use of "auto" outside of macros
+is discouraged.
+
 Especially with pointer types, whenever the pointed to object is not
 (supposed to be) modified, qualify the pointed to type with "const".