Serialize PCI device hierarchy and top-level attribute files per NUMA node

[atoniolo76]

Stacks off #13297 and based on #13114. Adds serialization of the PCI device hierarchy in sysfs collected from /sys/bus/pci/devices/, filtering for GPU, InfiniBand NICs, PCI bridge, and NVSwitch device classes only. Exposes NUMA topology files cpumap, cpulist, and distance from /sys/devices/system/node/node{N}/ for all nodes and includes the top-level aggregate files online, possible, has_cpu, has_memory, and has_normal_memory at /sys/devices/system/node/.

Tested and confirmed full line rate bandwidth:

Files added:

• pci_devices.go adds PCIDeviceAttr struct containing address, realpath, class, vendor, device, subsystem IDs, numa_node, local_cpus, and link speed/width information. Filters for GPU/NIC/Bridge/NVSwitch devices by pciClassRelevant(). Reads PCI device attribute files and walks through the /sys/bus/pci/devices/ directory starting at leaf devices and ending at ancestor bridges. Other plumbing for serializing the data and re-building the chroot sysfs tree.
• numa.go adds NUMANodeData struct for id, cpumap, cpulist, and distance information. Adds NUMAData structure for the online, possible, has_cpu, has_memory, has_normal_memory aggregate files. Reads from /sys/devices/system/node/node{N}/ per-node to build NUMANodeData. Other plumbing for serializing the data and re-building the node directories with the attribute files plus the aggregate files at the /sys/devices/system/node/ level.

Files modified:

• pkg/sentry/fsimpl/sys/sys.go adds PCIDevicesData and NUMAData fields to the InternalData struct. Calls newPCIDevicesSysfsEntries() and newNUMASysfsEntries().
• runsc/cmd/chroot.go adds pciDevicesUpdateChroot() and numaUpdateChroot() functions, which are called before chroot while host sysfs is still accessible and passed as args PCIDevicesData and NUMAData into boot.
• runsc/cmd/boot.go adds calls to DeserializePCIDevicesData() and DeserializeNUMAData() after pivot root.
• runsc/boot/loader.go adds pciDevicesData and numaData fields to the loader struct.
• runsc/boot/vfs.go passes pciDevicesData and numaData into mount options to make available for the sysfs filesystem constructor.

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