Fix resize ordering for COW VHD

When resizing, the snapshot created by snapshot_attached_here
is still required outside of the with block.  This works by coincidence
because XenServer in OS only supports ext3, which does not support
automatic coalescing by default.  This is in the process of being
changed.

Change-Id: I80fa02ea279839921218655cb6de4e88f595a65b
Merge "Fix resize ordering for COW VHD"