runsc: serialize RDMA and PCI sysfs data at boot

[atoniolo76]

Snapshot host RDMA/InfiniBand and PCI device sysfs attributes before pivot_root (while the host sysfs is still accessible), serialize them as JSON, and reconstruct them as virtual kernfs entries inside the sentry. This gives NCCL and libibverbs the topology information they need for device discovery and PCI distance computation without granting the sandbox access to the real host sysfs.

Key components:

• pkg/sentry/fsimpl/sys/rdma.go: RDMA device data collection and virtual /sys/class/infiniband{,_verbs}/ + /sys/class/net/ entries
• pkg/sentry/fsimpl/sys/pci_devices.go: PCI topology collection and virtual /sys/devices/pci*/ hierarchy with symlinks for NCCL
• runsc/cmd/chroot.go: collection before pivot_root
• runsc/cmd/boot.go: --rdmaproxy flag and data deserialization
• runsc/boot/vfs.go: wire sysfs InternalData for RDMA/PCI
• runsc/config: --rdmaproxy configuration flag

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