Firecracker snapshot upf ctriface

[char-1ee]

This is an informal PR for debugging. Please DO NOT merge.

1. These updates are based on my firecracker-snapshot-upf-test branch: https://github.com/char-1ee/vHive/tree/firecracker-snapshot-upf-test.
2. These updates currently cannot pass integration tests in ./vhive_test.go
3. I committed the updates that can pass ./ctriface/iface_test.go, which contains the TestStartSnapStop() from failing_test.go.

[char-1ee]

@ustiugov