x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set

In preparation for upcoming CET virtualization support, the CET supervisor
state will be added as a "guest-only" feature, since it is required only by
KVM (i.e., guest FPUs). Establish the infrastructure for "guest-only"
features.

Define a new XFEATURE_MASK_GUEST_SUPERVISOR mask to specify features that
are enabled by default in guest FPUs but not in host FPUs. Specifically,
for any bit in this set, permission is granted and XSAVE space is allocated
during vCPU creation. Non-guest FPUs cannot enable guest-only features,
even dynamically, and no XSAVE space will be allocated for them.

The mask is currently empty, but this will be changed by a subsequent
patch.

Co-developed-by: Chao Gao <[email protected]>
Signed-off-by: Chao Gao <[email protected]>
Signed-off-by: Yang Weijiang <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Rick Edgecombe <[email protected]>
Reviewed-by: John Allen <[email protected]>
Link: https://lore.kernel.org/all/20250522151031.426788-6-chao.gao%40intel.com

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