KVM: x86: hyper-v: Honor HV_MSR_VP_INDEX_AVAILABLE privilege bit

HV_X64_MSR_VP_INDEX is only available to guest when
HV_MSR_VP_INDEX_AVAILABLE bit is exposed.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-9-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>