KVM: x86: Have all vendor neutral sub-configs depend on KVM_X86, not just KVM

KVM: x86: Have all vendor neutral sub-configs depend on KVM_X86, not just KVM

Make all vendor neutral KVM x86 configs depend on KVM_X86, not just KVM,
i.e. gate them on at least one vendor module being enabled and thus on
kvm.ko actually being built.  Depending on just KVM allows the user to
select the configs even though they won't actually take effect, and more
importantly, makes it all too easy to create unmet dependencies.  E.g.
KVM_GENERIC_PRIVATE_MEM can't be selected by KVM_SW_PROTECTED_VM, because
the KVM_GENERIC_MMU_NOTIFIER dependency is select by KVM_X86.

Hiding all sub-configs when neither KVM_AMD nor KVM_INTEL is selected also
helps communicate to the user that nothing "interesting" is going on, e.g.

  --- Virtualization
  <M>   Kernel-based Virtual Machine (KVM) support
  < >   KVM for Intel (and compatible) processors support
  < >   KVM for AMD processors support

Fixes: ea4290d77bda ("KVM: x86: leave kvm.ko out of the build if no vendor module is requested")
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Message-ID: <20250729225455.670324-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>