Merge tag 'kvm-x86-mmio-7.1' of https://github.com/kvm-x86/linux into HEAD

Merge tag 'kvm-x86-mmio-7.1' of https://github.com/kvm-x86/linux into HEAD

KVM x86 emulated MMIO changes for 7.1

Copy single-chunk MMIO write values into a persistent (per-fragment) field to
fix use-after-free stack bugs due to KVM dereferencing a stack pointer after an
exit to userspace.

Clean up and comment the emulated MMIO code to try to make it easier to
maintain (not necessarily "easy", but "easier").