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
xenapi: post install scriptlet to rpm

A freshly installed XenServer requires some modifications in order to be
able to run OpenStack. Nova contains an rpm specification to create a
package with the dom0 plugins, however installing the rpm is not enough.
Some more actions need to be taken:
 - create /images, link to a directory within the local SR
 - setup an ssh keypair, and authenticate it with the host
This change adds a post script to the nova plugins rpm spec file, so
by installing the rpm on a XenServer, it is ready to run OpenStack, no
further manual actions needed.

Change-Id: Id57e3a3ab2782550468c6aa4c01059ceb7cc81ab