KVM selftests changes for 6.9:

 - Add macros to reduce the amount of boilerplate code needed to write "simple"
   selftests, and to utilize selftest TAP infrastructure, which is especially
   beneficial for KVM selftests with multiple testcases.

 - Add basic smoke tests for SEV and SEV-ES, along with a pile of library
   support for handling private/encrypted/protected memory.

 - Fix benign bugs where tests neglect to close() guest_memfd files.