Introduce new block stream test cases:

refactor block stream cancel, set speed to make
script more readable;

block_stream_simple test include sub-tests:

1) Cancel block stream job aync and sync, AKA
cancel before/after set block job speed event;
2) set limited speed when job start and reset
job speed after job started and query block job
status used in each test of block stream tests,
so no special test for it;
Signed-off-by: NXu Tian <xutian@redhat.com>