Intercept qemu exit port to achieve grace VM shutdown

[aarkegz]

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[Copilot]

Pull Request Overview

This PR refactors the handling of I/O port interception by introducing dedicated QEMU exit port constants for a graceful VM shutdown and removing the duplicate constant definitions from the VCPU exit handling logic. Key changes include:

• Adding new constants QEMU_EXIT_PORT and QEMU_EXIT_MAGIC at the top of the file.
• Updating the I/O bitmap setup to intercept the QEMU exit port.
• Removing duplicate QEMU exit definitions in the VCPU exit handling section.

Comments suppressed due to low confidence (1)

src/vmx/vcpu.rs:468

• Consider adding a brief comment explaining the intended use of QEMU_EXIT_PORT and QEMU_EXIT_MAGIC to clarify their role in achieving a graceful VM shutdown.

QEMU_EXIT_PORT..QEMU_EXIT_PORT + 1, // QEMU exit port