xenapi: support ephemeral disks bigger than 2TB

The VHD format, as implemented in XenAPI, has a hard limit
of 2043GB.

This change ensures if you request an ephemeral disk
greater than this size, multiple disks are attached
to fulfill the space requested in the flavor.

By default each disk is 2000 GB in size, unless
the size in the flavor is a multiple of 1024, then
each disk is 1024 GB in size.

Implements blueprint xenapi-large-ephemeral-disk-support
Change-Id: I98d928c022bf8f2ac3f6530aac398d5cc77cdb2a
Merge "xenapi: support ephemeral disks bigger than 2TB"