Backingchain: add new case for blockjob with --async

[meinaLi]

This PR adds: VIRT-294691 - Abort a block job with --async option

# avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 backingchain.blockjob.async
JOB ID     : 7606e551ada289234c8110fe7f05b9706ddf9528
JOB LOG    : /var/lib/avocado/job-results/job-2022-11-06T22.00-7606e55/job.log
 (1/1) type_specific.io-github-autotest-libvirt.backingchain.blockjob.async.async_option: PASS (71.53 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2022-11-06T22.00-7606e55/results.html
JOB TIME   : 75.02 s

Signed-off-by: Meina Li [email protected]

[meinaLi]

Labels: modular_redesign