memory-device: document MemoryDeviceClass

memory-device: document MemoryDeviceClass

Document the functions. Don't document get_region_size(), as we will be
dropping/replacing that one soon.

Use same documentation style as in include/exec/memory.h, but don't
document the parameters, as they are self-explanatory.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181005092024.14344-9-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>