xenapi: Avoid hotplugging volumes on resize.

In finish_migration, instead of starting up the vm and then
hotplugging in the volumes to it, just attach the volumes before
starting the vm.

This avoids the issue where the guest OS hasn't loaded the PV
drivers for the volume yet.

Fixes bug 1094351

Change-Id: I51d754f8f82f1d22bc123b39777449b58b03e389
Merge "xenapi: Avoid hotplugging volumes on resize."