vpci: add the ability to defer write results to configuration space

[mfrohlich-msft]

Currently, vpci bus mmio writes do not allow deferral of pci config space writes because an unwrap() call will panic on the deferred access.

This change allows emulated configuration space accesses for VPCI devices to defer their configuration space write requests to be completed asynchronously. If multiple chunked accesses result in deferred writes, collects the tasks and polls them to completion in a separate deferred task returned to the vmotherboard.

This retains the same expectation that all deferred writes must return IoResult::Ok or it panics.

[Copilot]

Pull request overview

This PR updates the VPCI bus MMIO config-space write path to support devices that return deferred completion for PCI config writes, by aggregating multiple deferred write tokens into a single deferred operation that vmotherboard can poll to completion.

Changes:

• Teach VpciBusDevice::mmio_write to handle IoResult::Defer from pci_cfg_write instead of panicking.
• Add a VmTaskDriver spawner to VpciBusDevice and spawn a task to await multiple deferred write tokens, completing an outer deferred write token.
• Add futures and pal_async dependencies to support polling and spawning.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 4 comments.

File	Description
vm/devices/pci/vpci/src/bus.rs	Collect deferred config-write tokens and return a combined deferred write to the motherboard, driven by a spawned task.
vm/devices/pci/vpci/Cargo.toml	Add futures and pal_async dependencies required by the new deferred-write combiner.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.