entity-manager: fixes for logDeviceAdded/Removed

Follow-up patch to fix issues found in comments of [1].

- Use explicit type rather than 'auto' for local variable
- Setup a return variable to avoid

entity-manager: fixes for logDeviceAdded/Removed

Follow-up patch to fix issues found in comments of [1].

- Use explicit type rather than 'auto' for local variable
- Setup a return variable to avoid duplicate initial value "Unknown"
- Fix string value read to avoid uncaught exception
- Remove unused boost include
- Add unit tests

Tested: Unit tests pass.

References:
[1] https://gerrit.openbmc.org/c/openbmc/entity-manager/+/84340

Change-Id: I3d5540860e8ef8e590bc2685ce559c53dc8452b5
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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