xen: remove workaround to inject evtchn_irq on irq enable

xen: remove workaround to inject evtchn_irq on irq enable

evtchn_upcall_pending is already set by common code at vcpu creation,
therefore on ARM we also need to call vgic_vcpu_inject_irq for it.
Currently we do that from vgic_enable_irqs as a workaround.

Do this properly by introducing an appropriate arch specific hook:
arch_evtchn_inject. arch_evtchn_inject is called by map_vcpu_info to
inject the evtchn irq into the guest. On ARM is implemented by calling
vgic_vcpu_inject_irq.

On x86 guests typically don't call VCPUOP_register_vcpu_info on vcpu0,
therefore avoiding the issue. However theoretically they could call
VCPUOP_register_vcpu_info on vcpu0 and in that case Xen would need to
inject the event channel notification into the guest if the guest is
HVM. So implement arch_evtchn_inject on x86 by calling
hvm_assert_evtchn_irq.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@linaro.org>