[Feature request] MDIO bus

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1874676

Various network devices open-code a MDIO bus with a dedicated PHY.
Introduce a new MDIO subsystem to
- reuse various duplicated components
- be able to interchange PHYs
- have common tracing
- use libqos to test all the PHYs from different NICs

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