xen: add support for canceled suspend

When running on Xen, it's possible that a suspend request to the hypervisor
fails (return from HYPERVISOR_suspend different than 0). This means that the
suspend hasn't succeed, and the resume procedure needs to properly handle this
case.

First of all, when such situation happens there's no need to reset the vector
callback, hypercall page, shared info, event channels or grant table, because
it's state is preserved. Also, the PV drivers don't need to be reset to the
initial state, since the connection with the backed has not been interrupted.

Submitted by: Liuyingdong <liuyingdong@huawei.com>
Reviewed by: royger
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D9635