xenapi: fix live-migrate with volume attached

We need to attach the volume SR on the destination host during
pre-live-migrate, so XenAPI spots the volume is present, and does't try
to block migrate the volume to the destination host, instead it just
connects to the remote volume before resuming the VM on the destination
host.

The fix is done by calling connect_volume instead of attach_volume. The
None passed into instance was causing the lookup of the vm_ref to
(correctly) fail, with:
Failure: ['FIELD_TYPE_ERROR', 'label']

Change-Id: I7eaf90582508b42a202da79072c891c216d137d1
Closes-Bug: #1317208
Merge "xenapi: fix live-migrate with volume attached"