lvm: disallow using non-empty volume groups
lvm: disallow using non-empty volume groups

This is in line with what we do for the other drivers. I might loosen this
restriction in a follow up commit when the following two conditions are met:
- the volume group uses a thin pool
- the thin pool is the only logical volume that exists on pool creation

 Closes #3351.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

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