KVM cleanups for 6.10:

 - Misc cleanups extracted from the "exit on missing userspace mapping" series,
   which has been put on hold in anticipation of a "KVM Userfault" approach,
   which should provide a superset of functionality.

 - Remove kvm_make_all_cpus_request_except(), which got added to hack around an
   AVIC bug, and then became dead code when a more robust fix came along.

 - Fix a goof in the KVM_CREATE_GUEST_MEMFD documentation.