XenAPI: make_partition fixes for Dom0

When running in dom0, the calls to parted fail to re-read the partition
table so we have to use kpartx to read it specifically.

The failure to re-read the partition table also means that the exit code
is non-0.

Implements blueprint xenserver-core

Change-Id: I56ca25874d6491b0ed70e5efd477254bfe47294d
Merge "XenAPI: make_partition fixes for Dom0"