KVM: arm64: Document PV-sched interface

virt inclusion
category: feature
bugzilla: 47624
CVE: NA

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Introduce a paravirtualization interface for KVM/arm64 to PV-sched.

A hypercall interface is provided for the guest to interrogate the
hypervisor's support for this interface and the location of the shared
memory structures.

Signed-off-by: Zengruan Ye <yezengruan@huawei.com>
Reviewed-by: Zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>