[incomplete] Share Eventchn, Gnt code with Mirage

Export new 'xen-event' and 'xen-grant' ocamlfind packages which contain the public parts of libxc, needed for block and network devices. Note the 'xenctrl' package still exists and has all the old stuff in it, for maximum backwards compatibility. 

Note there is a backwards incompatible change: renaming labelled arguments spelled 'writeable' with 'writable'. This standardises the spelling we're using. There are a few users which will need to be fixed up.

This allows all the libraries to be installed together: previously mirage-xen conflicted with this package since it replaced the 'xenctrl' package.

This is part of a larger set of changes: [mirage/mirage-platform#83]