Update xen-disk.1.2.0 to xen-disk.1.2.1, to better sync with wiki

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #18 from djs55/xen-disk.1.2.1

Update xen-disk.1.2.0 to xen-disk.1.2.1, to better sync with wiki