Add the block_stream command, which starts copy backing file contents
into the image file. Also add the BLOCK_JOB_COMPLETED QMP event which
is emitted when image streaming completes. Later patches add control
over the background copy speed, cancelation, and querying running
streaming operations.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Acked-by: Luiz Capitulino <lcapitulino@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>