KVM: Move KVM_VM_MEMORY_ATTRIBUTES config definition to x86

Bury KVM_VM_MEMORY_ATTRIBUTES in x86 to discourage other architectures
from adding support for per-VM memory attributes, because tracking private
vs. shared memory on a per-VM basis is now deprecated in favor of tracking
on a per-guest_memfd basis, and while RWX memory attributes are on the
horizon, they too are expected to be x86-only.

This will also allow modifying KVM_VM_MEMORY_ATTRIBUTES to be
user-selectable (in x86) without creating weirdness in KVM's Kconfigs.
Now that guest_memfd supports in-place conversions, it's entirely possible
to run x86 CoCo VMs without support for KVM_VM_MEMORY_ATTRIBUTES.

Leave the code itself in common KVM so that it's trivial to undo this
change if new per-VM attributes do come along.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260910-gmem-inplace-conversion-v13-21-dd6fbf94f4e1@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/Kconfig

@@ -80,6 +80,9 @@ config KVM_WERROR
 
 	  If in doubt, say "N".
 
+config KVM_VM_MEMORY_ATTRIBUTES
+	bool
+
 config KVM_SW_PROTECTED_VM
 	bool "Enable support for KVM software-protected VMs"
 	depends on EXPERT

virt/kvm/Kconfig

@@ -100,9 +100,6 @@ config KVM_ELIDE_TLB_FLUSH_IF_YOUNG
 config KVM_MMU_LOCKLESS_AGING
        bool
 
-config KVM_VM_MEMORY_ATTRIBUTES
-       bool
-
 config KVM_GUEST_MEMFD
        select XARRAY_MULTI
        bool