Add minimal plumbing to serialize sysfs information for ib_write_bw

[atoniolo76]

Based on #13114, this PR provides the minimal devices + sysfs information necessary to get ib_write_bw working between two gVisor containers (relies on later rdmaproxy PRs):

The serialized JSON fields per device contain basic device identity information like name, abi_version, node_type, node_guid, dev, ibdev, fw_ver, and sys_image_guid. Modalias provides the PCI information that libibverbs uses to match a device to a user space driver plugin (e.g. mlx5). PCI attributes about the NIC device include pci_slot_name, pci_driver, pci_class, pci_vendor, pci_device, pci_subsys_vendor, and pci_subsys_device. numa_node and local_cpulist detail which NUMA node and CPUs the NIC is closest to. Finally, the ports array includes various per-port information.

verbs_abi_version tells libibverbs which version of the kernel uverbs API is supported. This file lives globally in sys/class/infiniband_verbs/abi_version.

Files added:

• pkg/sentry/fsimpl/sys/rdma.go contain the RDMA-specific data-types, host sysfs collection, and interface for serialization -> deserialization -> reconstruction of virtual sysfs tree.
• runsc/specutils/rdma.go gates the entire serialization process on the presence of /dev/infiniband/uverbs* in the OCI spec.

Files modified:

• runsc/cmd/chroot.go executes collection of RDMA information from sysfs before pivot_root. Data is serialized to /var/lib/gvisor/rdma_data.json inside the chroot.
• runsc/cmd/boot.go deserializes the collected data from /var/lib/gvisor/rdma_data.json and is passed into the boot args RDMADevices.
• runsc/boot/loader.go stores the RDMADevices args on the Loader struct.
• runsc/boot/vfs.go passes rdmaDevices into sys.InternalData.
• pkg/sentry/fsimpl/sys/sys.go reads sys.InternalData and calls rdma.go to build the /sys/class/infiniband_verbs/ and /sys/class/infiniband/ trees.
• runsc/container/container.go tells the gofer to create RDMA devices when the OCI spec lists uverbs devices.
• runsc/cmd/sandboxsetup/gofer_mount.go adds ShouldExposeRDMADevice to the gofer's device filer for bind-mounting the uverbs device nodes when /dev/infiniband/uverbs* is included in the OCI spec.

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[trantoji]

Very Nice, thank you.