[vminitd]: api for freeze/thaw filesystem operations

[saehejkang]

Addition of vminitd API for freeze/thaw filesystem operations

Closes #660

[jglogan]

@dcantah wdyt?

[dcantah]

Looks good!

[saehejkang]

Marking this ready for review as it is mainly feature complete.

---

@dcantah @jglogan

Is there a proper way to test these changes? I don't see any testing patterns for the vminitd api and I am sure there are some kinks that I need to fix.

[jglogan]

@saehejkang Congrats! And it's a good question.

I'm wondering if you could make a copy of one of the really simple "run container and exec a command" integration tests and extend it so that it does this instead:

• create an ext4 disk image file with a blank filesystem on it, defer its deletion
• run the container, with the disk image mounted to /data
• run a freeze operation
• exec an echo hello > /data/hello.txt
• copy (clone) the disk image file, defer its deletion
• run a thaw operation
• stop the container
• run another container, with the clone mounted to '/data'
• exec dmesg, ensure that the /data filesystem does not need fsck and mounted cleanly
• exec an ls /data, ensure that hello.txt does not exist
• stop the container

[dcantah]

You'll need to run make fmt here

[saehejkang]

@jglogan @dcantah ready for another review 🙏🏽