KVM selftests fixes/cleanups (and one KVM x86 cleanup) for 6.8:

 - Remove redundant newlines from error messages.

 - Delete an unused variable in the AMX test (which causes build failures when
   compiling with -Werror).

 - Fail instead of skipping tests if open(), e.g. of /dev/kvm, fails with an
   error code other than ENOENT (a Hyper-V selftest bug resulted in an EMFILE,
   and the test eventually got skipped).

 - Fix TSC related bugs in several Hyper-V selftests.

 - Fix a bug in the dirty ring logging test where a sem_post() could be left
   pending across multiple runs, resulting in incorrect synchronization between
   the main thread and the vCPU worker thread.

 - Relax the dirty log split test's assertions on 4KiB mappings to fix false
   positives due to the number of mappings for memslot 0 (used for code and
   data that is NOT being dirty logged) changing, e.g. due to NUMA balancing.

 - Have KVM's gtod_is_based_on_tsc() return "bool" instead of an "int" (the
   function generates boolean values, and all callers treat the return value as
   a bool).