Fixes Hyper-V resize down exception

The Hyper-V driver does not support resize down and is currently
raising an exception if the user attempts to do that, causing the
instance to go in ERROR state.

The driver should use the recently introduced instance faults
"exception.InstanceFaultRollback" instead, which will leave the
instance in ACTIVE state as expected.

Closes-Bug: #1354448

Change-Id: Ibaf8482562094cd2b3165dc62a907fa9e0e56e19
Merge "Fixes Hyper-V resize down exception"