KVK generic changes for 6.12:

 - Fix a bug that results in KVM prematurely exiting to userspace for coalesced
   MMIO/PIO in many cases, clean up the related code, and add a testcase.

 - Fix a bug in kvm_clear_guest() where it would trigger a buffer overflow _if_
   the gpa+len crosses a page boundary, which thankfully is guaranteed to not
   happen in the current code base.  Add WARNs in more helpers that read/write
   guest memory to detect similar bugs.