[libvirt] 'build' on FS pool now unconditionally formats?

[Cole Robinson]

Hi guys,

Looking at the new FS pool build options and talking with Dave, I see that
calling PoolBuild on an FS pool now unconditionally calls mkfs. This is really
bad when mixed with virt-manager: previously, we assumed the FS build command
was always non destructive (at most it created a directory), so we called it
every time, and didn't even allow users to opt out, since there wasn't a use
case that called for it.

This new formatting behavior really needs to be opt in, otherwise all
virt-manager versions creating an FS pool can destroy data.

Just FYI, for disk pools (and certain LVM configurations) where this operation
has always been destructive, we default to build=off, and loudly warn the user
if they choose otherwise. We can do that with this new option as well, but the
previous behavior really needs to be reinstated IMO (and before the new release).

I fully accept that this could be a bug in virt-manager's assumptions of the
build command, but even consider a virsh user: previously build just created a
directory, now it formats a partition, without any XML change.

Thanks,
Cole