virtio-mem: support CONFIG_PROC_VMCORE_DEVICE_RAM

virtio-mem: support CONFIG_PROC_VMCORE_DEVICE_RAM

Let's implement the get_device_ram() vmcore callback, so
architectures that select NEED_PROC_VMCORE_NEED_DEVICE_RAM, like s390
soon, can include that memory in a crash dump.

Merge ranges, and process ranges that might contain a mixture of plugged
and unplugged, to reduce the total number of ranges.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20241204125444.1734652-12-david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>