tools/libxl: Initialise the fd of the unused half of a datacopier

tools/libxl: Initialise the fd of the unused half of a datacopier

This bug causes a spurious failure if stdin happens to be an
appropriately readable/writeable pipe which receives a POLLHUP

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>