Fixes active volume resizes

When extending a volume attached to an active nova server, the guest
should be stopped, the volume detached, extended, reattached and
then the guest should be restarted before the task status is reset
to NONE from RESIZING.

This fixes extending volumes attached to active nova servers to ensure a
proper resize is performed and the task status is set to NONE after all
resize operations complete successfully. Unit tests are also added.

Change-Id: I9a5785f539a2ee138ce1ded3cbd795ef91d66400
Closes-Bug: #1259976