virtio-mem logically plugs/unplugs memory within a sparse memory region
and notifies via the RamDiscardManager interface when parts become
plugged (populated) or unplugged (discarded).

Currently, we end up (via the two users)
1) zeroing all logically unplugged/discarded memory during TPM resets.
2) reading all logically unplugged/discarded memory when dumping, to
   figure out the content is zero.

1) is always bad, because we assume unplugged memory stays discarded
   (and is already implicitly zero).
2) isn't that bad with anonymous memory, we end up reading the zero
   page (slow and unnecessary, though). However, once we use some
   file-backed memory (future use case), even reading will populate memory.

Let's cut out all parts marked as not-populated (discarded) via the
RamDiscardManager. As virtio-mem is the single user, this now means that
logically unplugged memory ranges will no longer be included in the
dump, which results in smaller dump files and faster dumping.

virtio-mem has a minimum granularity of 1 MiB (and the default is usually
2 MiB). Theoretically, we can see quite some fragmentation, in practice
we won't have it completely fragmented in 1 MiB pieces. Still, we might
end up with many physical ranges.

Both, the ELF format and kdump seem to be ready to support many
individual ranges (e.g., for ELF it seems to be UINT32_MAX, kdump has a
linear bitmap).

Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Claudio Fontana <cfontana@suse.de>
Cc: Thomas Huth <thuth@redhat.com>
Cc: "Alex Bennée" <alex.bennee@linaro.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Laurent Vivier <lvivier@redhat.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210727082545.17934-5-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>