KVM: nVMX: Update vmcs12 on BNDCFGS write, not at vmcs02=>vmcs12 sync

Update vmcs12->guest_bndcfgs on intercepted writes to BNDCFGS from L2
instead of waiting until vmcs02 is synchronized to vmcs12.  KVM always
intercepts BNDCFGS accesses, so the only way the value in vmcs02 can
change is via KVM's explicit VMWRITE during emulation.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220614215831.3762138-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>