virtio_transitional: add sleep time in virtio_transitional_rng

[meinaLi]

Fix the timeout expired issue by adding a sleep time to get the expected command result.

[meinaLi]

Test in local machine and passed in 20 times:

# for i in {1..20}; do echo "------$i------"; avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 virtio_transitional_rng.rhel6_guest.with_pcie_to_pci_bridge.boot_test.virtio; done
------1------
JOB ID     : d1a6491905d210feb2747afff641d50f49ce861c
JOB LOG    : /var/lib/avocado/job-results/job-2023-09-13T00.41-d1a6491/job.log
 (1/1) type_specific.io-github-autotest-libvirt.virtio_transitional_rng.rhel6_guest.with_pcie_to_pci_bridge.boot_test.virtio: PASS (136.04 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2023-09-13T00.41-d1a6491/results.html
JOB TIME   : 137.82 s
......
------20------
JOB ID     : ea765b7f8128dc98ba43ea7a0b932f6a04dfff2b
JOB LOG    : /var/lib/avocado/job-results/job-2023-09-13T01.29-ea765b7/job.log
 (1/1) type_specific.io-github-autotest-libvirt.virtio_transitional_rng.rhel6_guest.with_pcie_to_pci_bridge.boot_test.virtio: PASS (157.86 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2023-09-13T01.29-ea765b7/results.html
JOB TIME   : 159.67 s

Because the reproduce rate in auto job is relatively high, I retested it in jenkin and wait for checking again.