Unsetting size property does not change block based volume's sizes

[hamistao]

Summary

Unsetting size never resizes block based volumes filesystem volumes and block type volumes.

This applies both to instance volumes, by unsetting the size on the disk device through lxc config device unset inst root size, and to custom volumes, by unsetting its size property through lxc storage volume unset pool vol size. In both cases, setting such property doesn't resize the volume and only unsets the config key, masking the instance's real size.

Although a logical behavior, this could raise a lot of complexity:

Further more because of the delayed nature of volume resizes when applied to a profile of running instances, this unexpected behaviour would take effect at next reboot, which might be quite a while after the change was made.

For this reason I dont think we should apply this change at this time.

Also, block volumes currently can not be shrunk this would only really be useful in a handful of scenarios. This issue aims to raise discussion on whether we want to keep this behavior or we want to resize volumes in such scenarios. This PR has a proposed method to resize on such cases, assuming we want to go that route.