xenstore: fix suspension when using the xenstore device

Lock the xenstore request mutex when suspending user-space processes, in order
to prevent any process from holding this lock when going into suspension, or
else the xenstore suspend process is going to deadlock.

Submitted by: Liuyingdong <liuyingdong@huawei.com>
Reviewed by: royger
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D9638