provisioning (applehv): expand image and CoW tip

[eriksjolund]

Add info about how to expand the image with truncate

Add tip about using cp -c to save disk space

Rationale

I think the information could be useful.

Some extra info that is not included in the PR:
This how I usually handle an image file.

1. Download image.raw.gz
2. Uncompress

   gunzip image.raw.gz
   

3. Create dir

   mkdir backup
   

4. Create Copy-on-Write clone

   cp -c image.raw backup
   

5. Expand image

   truncate -c -s +10G image.raw
   

6. Start the vm with vfkit

[travier]

Thanks, some nits but sounds like a good tip.

[eriksjolund]

I applied all changes requested (I hope).

I also removed the word downloaded from this sentence

TIP: If you want to keep a copy of the downloaded image file, use `cp` with the `-c` flag.

as I wanted the recommendation to be about taking a backup of the decompressed image.