mirage-block-xen.1.1.0 and xen-disk.1.2.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
mirage-block-xen needs V1_LWT, so it needs mirage-types && ipaddr

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #16 from djs55/block

mirage-block-xen.1.1.0 and xen-disk.1.2.0