KVM: selftests: Test READ=>WRITE dirty logging behavior for shadow MMU

KVM: selftests: Test READ=>WRITE dirty logging behavior for shadow MMU

Update the nested dirty log test to validate KVM's handling of READ faults
when dirty logging is enabled.  Specifically, set the Dirty bit in the
guest PTEs used to map L2 GPAs, so that KVM will create writable SPTEs
when handling L2 read faults.  When handling read faults in the shadow MMU,
KVM opportunistically creates a writable SPTE if the mapping can be
writable *and* the gPTE is dirty (or doesn't support the Dirty bit), i.e.
if KVM doesn't need to intercept writes in order to emulate Dirty-bit
updates.

To actually test the L2 READ=>WRITE sequence, e.g. without masking a false
pass by other test activity, route the READ=>WRITE and WRITE=>WRITE
sequences to separate L1 pages, and differentiate between "marked dirty
due to a WRITE access/fault" and "marked dirty due to creating a writable
SPTE for a READ access/fault".  The updated sequence exposes the bug fixed
by KVM commit 1f4e5fc83a42 ("KVM: x86: fix nested guest live migration
with PML") when the guest performs a READ=>WRITE sequence with dirty guest
PTEs.

Opportunistically tweak and rename the address macros, and add comments,
to make it more obvious what the test is doing.  E.g. NESTED_TEST_MEM1
vs. GUEST_TEST_MEM doesn't make it all that obvious that the test is
creating aliases in both the L2 GPA and GVA address spaces, but only when
L1 is using TDP to run L2.

Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20260115172154.709024-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>