tools/xenconsoled: Initialise pointers before trying to use them

tools/xenconsoled: Initialise pointers before trying to use them

This is a regression introduced by

"Switch from select() to poll() in xenconsoled's IO loop."
  hg c/s 26405:7359c3122c5d
  git cc5434c933153c4b8812d1df901f8915c22830a8

which results in reliable segfaults during VM power operations.

Switch to calloc(3) in an effort to avoid similar problems with changes in the
future.

Signed-off-by: Marcus Granado <marcus.granado@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>