xen/xenbus: fix W=1 build warning in xenbus_va_dev_error function

This patch fixes a W=1 format-string warning reported by GCC 12.3.0
by annotating xenbus_switch_fatal() and xenbus_va_dev_error()
with the __printf attribute. The attribute enables compile-time
validation of printf-style format strings in these functions.

The original warning trace:
drivers/xen/xenbus/xenbus_client.c:304:9: warning: function 'xenbus_va_dev_error' might be
a candidate for 'gnu_printf' format attribute [-Wsuggest-attribute=format]

Signed-off-by: Peng Jiang <jiang.peng9@zte.com.cn>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250620084104786r5xoR16_AmYZMJLnno3_Q@zte.com.cn>
Signed-off-by: Juergen Gross <jgross@suse.com>