KVM: nVMX: Use KVM-governed feature framework to track "nested VMX enabled"

KVM: nVMX: Use KVM-governed feature framework to track "nested VMX enabled"

Track "VMX exposed to L1" via a governed feature flag instead of using a
dedicated helper to provide the same functionality.  The main goal is to
drive convergence between VMX and SVM with respect to querying features
that are controllable via module param (SVM likes to cache nested
features), avoiding the guest CPUID lookups at runtime is just a bonus
and unlikely to provide any meaningful performance benefits.

Note, X86_FEATURE_VMX is set in kvm_cpu_caps if and only if "nested" is
true, and the CPU obviously supports VMX if KVM+VMX is running.  I.e. the
check on "nested" is now implicitly down by the kvm_cpu_cap_has() check
in kvm_governed_feature_check_and_set().

No functional change intended.

Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Reviwed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230815203653.519297-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>