KVM: x86: Merge 'selftests' into 'cet' to pick up ex_str()

Merge the queue of KVM selftests changes for 6.18 to pick up the ex_str()
helper so that it can be used to pretty print expected versus actual
exceptions in a new MSR selftest.  CET virtualization will add support for
several MSRs with non-trivial semantics, along with new uAPI for accessing
the guest's Shadow Stack Pointer (SSP) from userspace.

diff --cc tools/testing/selftests/kvm/x86/pmu_counters_test.c
Simple merge