xenapi: migrate multiple ephemeral disks

Current migrate does not move ephemeral disks. This change allows those
disks to be moved. Note there could be several ephemeral disks in the
case where there are ephemeral disks that would be too big for a single
VHD file, or split up to improve performance.

This change disables the ability to resize ephemeral disks. A later
patch will look at enabling resize-up for ephemeral disks. Currently,
during resize, the ephemeral disks are just dropped and re-recreated.

Implements blueprint xenapi-migrate-ephemeral-disks
Change-Id: I8d7d84b54d2a876a510c442bb11a1c64c35feb00