DAOS-18487 utils: refine ddb_iov_to_printable_buf

[liuxuezhao]

Current code may cause some integer keys' output not-readable, for example dkey 10, 60 printed by ddb as -
DKEY: (/[4]/[2]/[0]) /b2b37b1a-2894-4970-b51c-1a964526bb02/948289433254841843.256.535.2/ {8}
DKEY: (/[4]/[2]/[2]) /b2b37b1a-2894-4970-b51c-1a964526bb02/948289433254841843.256.535.2/<{8}

That cause confuse and misunderstanding of ddb output.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Errors are Unable to load ticket data
https://daosio.atlassian.net/browse/DAOS-18487

[daosbuild3]

Test stage Unit Test on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17594/1/testReport/

[daosbuild3]

Test stage Unit Test with memcheck on EL 8.8 completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17594/1/testReport/

[Nasf-Fan]

This patch is workable for integer key, but maybe generate wrong output for sting formatted key. I will make more suitable solution via DAOS-18625.

[Nasf-Fan]

This patch is workable for integer key, but maybe generate wrong output for sting formatted key. I will make more suitable solution via DAOS-18625.

Here is the patch:
#17617