minios: reverse layering of xc vs minios fd close

minios: reverse layering of xc vs minios fd close

Having minios close() call back into the libxc core close routines is
backwards and unexpected. On every other OS the libxc core close
routine calls close().

Export minios specific functions from the minios libxc code to
implement fd closing for each type of xc file handle and simply call
close() in the core close routine.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>