xen/timer: mark the Xen PV timer as not safe for suspension

Note that the timer itself fully supports suspension, but due to the lack of
ordering during the resume process FreeBSD cannot guarantee that the timer is
resumed before any device attempts to use it.

Submitted by: Liuyingdong <liuyingdong@huawei.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D9639