Update for xen-disk 1.2.1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
bumped xen-disk to 1.2.1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
s/xen/Xen/g before the branding police arrive

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #93 from djs55/update-xen-disk

Update for xen-disk 1.2.1