KVM: x86/xen: handle PV timers oneshot mode

KVM: x86/xen: handle PV timers oneshot mode

If the guest has offloaded the timer virq, handle the following
hypercalls for programming the timer:

    VCPUOP_set_singleshot_timer
    VCPUOP_stop_singleshot_timer
    set_timer_op(timestamp_ns)

The event channel corresponding to the timer virq is then used to inject
events once timer deadlines are met. For now we back the PV timer with
hrtimer.

[ dwmw2: Add save/restore, 32-bit compat mode, immediate delivery,
         don't check timer in kvm_vcpu_has_event() ]

Signed-off-by: Joao Martins <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20220303154127[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>