storage_ceph: use xargs instead of parallel as a tool of parallel testing

[meinaLi]

Parallel package sometimes is not included in the new compose. So use xargs to have a stable test for this case.

[meinaLi]

# avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 virsh.pool.positive_test.pool_type_rbd
JOB ID     : 53912c117599a6e192b21a1ff42e12669988ee44
JOB LOG    : /var/log/avocado/job-results/job-2024-06-20T04.43-53912c1/job.log
 (1/1) type_specific.io-github-autotest-libvirt.virsh.pool.positive_test.pool_type_rbd: PASS (28.39 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-06-20T04.43-53912c1/results.html
JOB TIME   : 28.70 s

[meinaLi]

@chunfuwen Could you help to review this PR? Thanks.