xenapi: resize up ephemeral disks

After adding the ability to migrate ephemeral disks, we were unable to
perform any resize of ephemeral disks.

This change implements the ability to increase the size of ephemeral
disks, if that is required be the selected flavor.

Unlike with the root disk, we attempt to expand the partition and
filesystem will never be touched by the resize up process.

However, after this change, you will still not be able to resize down
the ephemeral disks. This function has proved very error prone when
dealing with root disks, so we are not implementing this feature for
ephemeral disks, and will error on requests to reduce the size of
ephemeral disks.

Implements blueprint xenapi-resize-ephemeral-disks
Change-Id: I218e9db9f5c912335c96da9831b300513213d33d
Merge "xenapi: resize up ephemeral disks"