x86/vvmx: don't enable vmcs shadowing for nested guests

Running "./xtf_runner vvmx" in L1 Xen under L0 Xen produces the
following result on H/W with VMCS shadowing:

    Test: vmxon
    Failure in test_vmxon_in_root_cpl0()
      Expected 0x8200000f: VMfailValid(15) VMXON_IN_ROOT
           Got 0x82004400: VMfailValid(17408) <unknown>
    Test result: FAILURE

This happens because SDM allows vmentries with enabled VMCS shadowing
VM-execution control and VMCS link pointer value of ~0ull. But results
of a nested VMREAD are undefined in such cases.

Fix this by not copying the value of VMCS shadowing control from vmcs01
to vmcs02.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

xen/arch/x86/hvm/vmx/vvmx.c

@@ -633,6 +633,7 @@ void nvmx_update_secondary_exec_control(struct vcpu *v,
                     SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY;
 
     host_cntrl &= ~apicv_bit;
+    host_cntrl &= ~SECONDARY_EXEC_ENABLE_VMCS_SHADOWING;
     shadow_cntrl = get_vvmcs(v, SECONDARY_VM_EXEC_CONTROL);
 
     /* No vAPIC-v support, so it shouldn't be set in vmcs12. */