This used to work in earlier version but was broken during a earlier
refactor.
This commit restores the functionality, but is also adds a boolean
per storage pool type if it supports overprovisioning.
This removes the nasty if which hard codes it.
UnitTests are also added to this commit to verify this functionality
remains stable in future releases.
Signed-off-by: Wido den Hollander <wido@widodh.nl>
