block: Introduce API for copy offloading

Introduce the bdrv_co_copy_range() API for copy offloading.  Block
drivers implementing this API support efficient copy operations that
avoid reading each block from the source device and writing it to the
destination devices.  Examples of copy offload primitives are SCSI
EXTENDED COPY and Linux copy_file_range(2).

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180601092648.24614-2-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>