NOT FOR UPSTEAM: Add 'magic' support

NOT FOR UPSTEAM: Add 'magic' support

Add a new XcMagic() function to xencontrol.dll and an example application
('magic') that will link to it and call it.

The function takes a UNC path as input, writes it to a xentore key
(/local/domain/X/data/unc-path), and then waits for a value to be
written to another xenstore key (/local/domain/X/data/device-id). That
value (interpreted as a numeric) is passed back to the caller.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>