CA-130161: Revert "CA-126097: Block clone of live VDIs"

Disks attached to a checkpoint look like they're live, so blocking clone
of live VDIs broke checkpoint revert.

This reverts commit a8aece189b38095c74a6e3b0aee3d8d255e42248.
CA-130161: Fix unit tests to test new behaviour

We've re-enabled clone of live VDIs, so the test needs to check this
instead of expecting failure.

Asserting whether a VDI can be cloned checks whether the backend
supports VDI_CLONE, so register the dummy SR with a dummy backend which
has a sensible set of features.

Signed-off-by: John Else <john.else@citrix.com>