pci_core: add infrastructure for devices to advertise pcie support with some basic functionality (#1930)

Adds the PCI_Express capability option for devices to advertise PCIe
support. This change will also us to add FLR support to the nvme_test
crate for further perf testing.

PciExpressCapability: New capability struct implementing PCI Express
Device Capabilities, Device Control, and Device Status registers

Spec digarams (From PCIe Base Spec 6.4) for reference:
<img width="1481" height="991" alt="image"
src="https://github.com/user-attachments/assets/a6f49e62-af03-4573-ab60-17637154aadc"
/>
<img width="1489" height="919" alt="image"
src="https://github.com/user-attachments/assets/21f6996c-eb8b-4fb6-b505-a113c37d9257"
/>

Being made as a helper PR to #1858 since that was getting a little too
large/complicated

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: Matt LaFayette (Kurjanowicz) <[email protected]>

Author: 3 people

vm/devices/pci/pci_core/src/capabilities/mod.rs

@@ -9,6 +9,7 @@ use inspect::Inspect;
 use vmcore::save_restore::ProtobufSaveRestore;
 
 pub mod msix;
+pub mod pci_express;
 pub mod read_only;
 
 /// A generic PCI configuration space capability structure.