xenapi: fix spawn servers with ephemeral disks

The new resize ephemeral disk code uses vdis["ephemerals"] to pass the
list of ephemeral vdis to the resize disk code. This is shared between
spawn and resize. As spawn wants to resize the root disk, resize=True
during the spawn operation.

The code in _attach_disks correctly deals with empty ephemeral disks,
but the code in _resize_up_vdis did not, and failed due to
ephemeral_vdis being None.

This is a quick fix for the issue. We need some refactoring in this area
so there is, ideally, only one place that generates ephemeral disks.

Change-Id: If058c5c17c2ed76569fd98d4a3d8e5274721fbd0
Closes-Bug: #1289361
Merge "xenapi: fix spawn servers with ephemeral disks"