Allow shrinking storage volumes

[marmarek]

While proper resizing filesystem first is tricky and not planned to be supported anytime soon, there are still cases where shrinking volume can be useful. For example installing/reinstalling template with root volume smaller than default one. This is especially the case for unikernel based systems.

Related: #3169 (comment)

[SuzanneSoy]

I also tried to shrink a VM's disk (in my case the private.img) immediately after creating it. It is possible, but the system does not recognize it happening:

$ qvm-volume resize --force dummytest:private "$((1024*1024*1024))"
For your own safety, shrinking of private is disabled. If you really know what you are doing, use `truncate` on appvms/dummytest/private manually.
$ truncate -s 1G /var/lib/qubes/appvms/dummytest/private.img && echo ok.
ok.
$ qvm-volume resize --force dummytest:private "$((1024*1024*1024))"
For your own safety, shrinking of private is disabled. If you really know what you are doing, use `truncate` on appvms/dummytest/private manually.
$ qvm-volume info dummytest:private size
2147483648 # Still the original size of 2 GiB
$ qvm-volume config dummytest:private size "$((1024*1024*1024))"
Invalid property: size

After that point, if I start the VM (i.e. its first boot) it seems that df -h reports the correct size within the VM, but qvm-volume does not detect the new, shrinked size.

What's worse is that if I make a backup of that VM, delete the original, restore it and start it, df -h shows that the disk image has been enlarged to the default size (2 GiB). I suppose that it would be possible to truncate the private.img file just after restoring the backup, but it seems hazardous to do so since at that point there is an actual filesystem on-disk (so putting the wrong size, say 1000*1000*1000 instead of 1024*1024*1024 would likely wreck the data).