This patch adds a test suite for the image streaming feature.  It
exercises the 'block_stream', 'block_job_cancel', 'block_job_set_speed',
and 'query-block-jobs' QMP commands.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>