cheripte: Make PTE bits reserved in guest page tables

The base hypervisor spec says that the format for guest
page table PTEs is the same as for normal PTEs and reserves
bits (e.g. the G bit) that don't have a meaning for guest
address translation (yet). Do the same for the cheripte
bits.

src/cheri-pte-ext.adoc

@@ -82,7 +82,9 @@ Generation (CRG), are added to leaf PTEs in Sv39, Sv48 and Sv57 as shown in
 xref:sv39pte[xrefstyle=short], xref:sv48pte[xrefstyle=short] and
 xref:sv57pte[xrefstyle=short] respectively. For non-leaf PTEs these bits
 remain reserved and must be cleared by software for forward compatibility,
-or else a page-fault exception is raised.
+or else a page-fault exception is raised. Additionally, if the hypervisor
+extension is enabled these bits remain reserved for leaf and non-leaf PTEs
+used in guest address translation.
 
 .Sv39 page table entry
 [#sv39pte]