xen: Switch to use kmemdup() helper

Use kmemdup() helper instead of open-coding to
simplify the code.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Chen Jiahao <chenjiahao16@huawei.com>
Link: https://lore.kernel.org/r/20230815092434.1206386-1-ruanjinjie@huawei.com


Signed-off-by: Juergen Gross <jgross@suse.com>