KVM: x86: disable preemption while updating apicv inhibition

Currently nothing prevents preemption in kvm_vcpu_update_apicv.

On SVM, If the preemption happens after we update the
vcpu->arch.apicv_active, the preemption itself will
'update' the inhibition since the AVIC will be first disabled
on vCPU unload and then enabled, when the current task
is loaded again.

Then we will try to update it again, which will lead to a warning
in __avic_vcpu_load, that the AVIC is already enabled.

Fix this by disabling preemption in this code.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220606180829.102503-6-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>