vmm: add NVIDIA GPUDirect P2P support

On platforms where PCIe P2P is supported, inject a PCI capability into
NVIDIA GPU to indicate support.

Signed-off-by: Thomas Barrett <tbarrett@crusoeenergy.com>

pci/src/vfio_user.rs

@@ -94,6 +94,7 @@ impl VfioUserPciDevice {
             &PciVfioUserSubclass::VfioUserSubclass,
             bdf,
             vm_migration::snapshot_from_id(snapshot.as_ref(), VFIO_COMMON_ID),
+            None,
         )
         .map_err(VfioUserPciDeviceError::CreateVfioCommon)?;