KVM: selftests: Verify KVM correctly handles mprotect(PROT_READ)

KVM: selftests: Verify KVM correctly handles mprotect(PROT_READ)

Add two phases to mmu_stress_test to verify that KVM correctly handles
guest memory that was writable, and then made read-only in the primary MMU,
and then made writable again.

Add bonus coverage for x86 and arm64 to verify that all of guest memory was
marked read-only.  Making forward progress (without making memory writable)
requires arch specific code to skip over the faulting instruction, but the
test can at least verify each vCPU's starting page was made read-only for
other architectures.

Link: https://lore.kernel.org/r/20241128005547.4077116-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>