Merge branch 'guest-memfd-mmap' into HEAD

Add support for host userspace mapping of guest_memfd-backed memory for VM
types that do NOT use support KVM_MEMORY_ATTRIBUTE_PRIVATE (which isn't
precisely the same thing as CoCo VMs, since x86's SEV-MEM and SEV-ES have
no way to detect private vs. shared).

mmap() support paves the way for several evolving KVM use cases:

* Allows VMMs like Firecracker to run guests entirely backed by
  guest_memfd [1]. This provides a unified memory management model for
  both confidential and non-confidential guests, simplifying VMM design.

* Enhanced Security via direct map removal: When combined with Patrick's
  series for direct map removal [2], this provides additional hardening
  against Spectre-like transient execution attacks by eliminating the
  need for host kernel direct maps of guest memory.

* Lays the groundwork for *restricted* mmap() support for guest_memfd-backed
  memory on CoCo platforms [3] that permit in-place sharing of guest memory
   with the host.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>