serial: Fix reset when disconnected

[Steven Malis (smalis-msft)]

Refactors how serial device state is initialized and reset to ensure that the device's modem connection state actually matches the backend connection status after a reset. This prevents devices from incorrectly reporting a carrier when no backend is attached. Also add new tests to verify this behavior.

[Copilot]

Pull request overview

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Refactors serial device initialization/reset so modem/carrier state reflects whether a backend is actually connected after reset, preventing false carrier reporting on disconnected ports, and adds regression tests.

Changes:

• Initialize State with backend connectivity (io.is_connected()) instead of always “connecting” after reset.
• Remove unconditional connect() calls in constructors and reset paths for PL011 and 16550.
• Add async tests ensuring reset does not assert carrier/modem lines when the backend is disconnected.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
vm/devices/serial/serial_pl011/src/lib.rs	Makes state init/reset conditional on backend connectivity; adds PL011 regression test for disconnected reset.
vm/devices/serial/serial_16550/src/lib.rs	Makes state init/reset conditional on backend connectivity; adds 16550 regression test for disconnected reset.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[github-actions]

3 Petri tests failed (0 unstable)