run unittests in privileged docker container

[roypat]

The code coverage tests already run in a privileged container, so this does not affect our security posture. Additionally, it will unblock vsock unit tests in the vhost-user-vsock crate, see also rust-vmm/vhost-device#706 and rust-vmm/vhost-device#728

Summary of the PR

Please summarize here why the changes in this PR are needed.

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• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[TimePrinciple]

I was about to ask if we could do the same for rust-vmm-container, I'll need to mount directories inside container to create images since some tests would fail in 9p rootfs as of riscv64.

I will bring up this discussion recently :)