DAOS-18296 rdb: Call vos_pool_create/open in ULTs (#17302)

[liw]

Call vos_pool_create and vos_pool_open in new deep-stack ULTs on the xstream to avoid pmemobj_create and pmemobj_open from potentially overflowing caller stacks.

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]

Ticket title is 'dmg system start causes engines to crash after pool create'
Status is 'In Progress'
Labels: 'request_for_2.6.5,scrubbed_2.8'
https://daosio.atlassian.net/browse/DAOS-18296

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