Initial versions of 'xen-disk' and dependencies

xen-disk:           serve a disk to a running VM tool
vhd-format:         reads and writes .vhd disk images
xenstore_transport: low-level xenstore read/write
xenctrl:            low-level xen hypercalls
xen-block-driver:   xen virtual disk protocol implementations
Merge pull request #916 from djs55/add-xen-disk

Initial versions of 'xen-disk' and dependencies