KVM: selftests: aarch64: Add readonly memslot tests into page_fault_test

Add some readonly memslot tests into page_fault_test. Mark the data and/or
page-table memory regions as readonly, perform some accesses, and check
that the right fault is triggered when expected (e.g., a store with no
write-back should lead to an mmio exit).

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221017195834.2295901-14-ricarkol@google.com