KVM: x86: Explicitly track feature flags that are enabled at runtime

Add one last (hopefully) CPUID feature macro, RUNTIME_F(), and use it
to track features that KVM supports, but that are only set at runtime
(in response to other state), and aren't advertised to userspace via
KVM_GET_SUPPORTED_CPUID.

Currently, RUNTIME_F() is mostly just documentation, but tracking all
KVM-supported features will allow for asserting, at build time, take),
that all features that are set, cleared, *or* checked by KVM are known to
kvm_set_cpu_caps().

No functional change intended.

Link: https://lore.kernel.org/r/20241128013424.4096668-57-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>