virtio-mem: support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE

The initial virtio-mem spec states that while unplugged memory should not
be read, the device still has to allow for reading unplugged memory inside
the usable region. The primary motivation for this default handling was
to simplify bringup of virtio-mem, because there were corner cases where
Linux might have accidentially read unplugged memory inside added Linux
memory blocks.

In the meantime, we:
1. Removed /dev/kmem in commit bbcd53c9 ("drivers/char: remove
   /dev/kmem for good")
2. Disallowed access to virtio-mem device memory via /dev/mem in
   commit 2128f4e2 ("virtio-mem: disallow mapping virtio-mem memory via
   /dev/mem")
3. Sanitized access to virtio-mem device memory via /proc/kcore in
   commit 0daa322b ("fs/proc/kcore: don't read offline sections,
   logically offline pages and hwpoisoned pages")
4. Sanitized access to virtio-mem device memory via /proc/vmcore in
   commit ce281462 ("virtio-mem: kdump mode to sanitize /...