util: rename qemu_open() to qemu_open_old()

We want to introduce a new version of qemu_open() that uses an Error
object for reporting problems and make this it the preferred interface.
Rename the existing method to release the namespace for the new impl.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

os-posix.c

@@ -297,7 +297,7 @@ void os_setup_post(void)
             error_report("not able to chdir to /: %s", strerror(errno));
             exit(1);
         }
-        TFR(fd = qemu_open("/dev/null", O_RDWR));
+        TFR(fd = qemu_open_old("/dev/null", O_RDWR));
         if (fd == -1) {
             exit(1);
         }