aarch64 haskell: implement isValidVTableRoot

The C code has an unnecessary name indirection via isValidNativeRoot
here, which I replicated to make more obvious what maps to what.
Eventually this should disappear.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

spec/haskell/src/SEL4/Kernel/VSpace/AARCH64.hs

@@ -458,17 +458,24 @@ getHWASID asid = do
 
 {- Helper Functions -}
 
+isVTableRoot :: Capability -> Bool
+isVTableRoot (ArchObjectCap (PageTableCap { capPTTopLevel = True })) = True
+isVTableRoot _ = False
+
+-- FIXME AARCH64: name indirection kept here for sync with C; both (C and
+-- Haskell) should define isValidVTableRoot directly
+isValidNativeRoot :: Capability -> Bool
+isValidNativeRoot cap = isValidVTableRoot cap && isJust (capPTMappedAddress (capCap cap))
+
+isValidVTableRoot :: Capability -> Bool
+isValidVTableRoot = isValidNativeRoot
+
 checkValidIPCBuffer :: VPtr -> Capability -> KernelF SyscallError ()
 checkValidIPCBuffer vptr (ArchObjectCap (FrameCap {capFIsDevice = False})) = do
     when (vptr .&. mask ipcBufferSizeBits /= 0) $ throw AlignmentError
     return ()
 checkValidIPCBuffer _ _ = throw IllegalOperation
 
-isValidVTableRoot :: Capability -> Bool
-isValidVTableRoot
-    (ArchObjectCap (PageTableCap { capPTMappedAddress = Just _ })) = True
-isValidVTableRoot _ = False
-
 maskVMRights :: VMRights -> CapRights -> VMRights
 maskVMRights r m = case (r, capAllowRead m, capAllowWrite m) of
     (VMReadOnly, True, _) -> VMReadOnly