KVM: nVMX: Prepare for enabling CET support for nested guest

KVM: nVMX: Prepare for enabling CET support for nested guest

Set up CET MSRs, related VM_ENTRY/EXIT control bits and fixed CR4 setting
to enable CET for nested VM.

vmcs12 and vmcs02 needs to be synced when L2 exits to L1 or when L1 wants
to resume L2, that way correct CET states can be observed by one another.

Please note that consistency checks regarding CET state during VM-Entry
will be added later to prevent this patch from becoming too large.
Advertising the new CET VM_ENTRY/EXIT control bits are also be deferred
until after the consistency checks are added.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Xin Li (Intel) <xin@zytor.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-32-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>