KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds

KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds

Enable KVM_GUEST_MEMFD for all KVM x86 64-bit builds, i.e. for "default"
VM types when running on 64-bit KVM.  This will allow using guest_memfd
to back non-private memory for all VM shapes, by supporting mmap() on
guest_memfd.

Opportunistically clean up various conditionals that become tautologies
once x86 selects KVM_GUEST_MEMFD more broadly.  Specifically, because
SW protected VMs, SEV, and TDX are all 64-bit only, private memory no
longer needs to take explicit dependencies on KVM_GUEST_MEMFD, because
it is effectively a prerequisite.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>