pc: rename "hotplug memory" terminology to "device memory"

Let's make it clear that we are dealing with device memory. That it can
be used for memory hotplug is just a special case.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180423165126.15441-10-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>