libxl: rename xenstored pid file on Linux

libxl: rename xenstored pid file on Linux

Changset 22806:eefb8e971be5 broke 'xl'.

After starting the xenstored, xenconsoled, xenbackendd and running xl
create I get this:
  libxl: error: libxl.c:53:libxl_ctx_init Is xenstore daemon running?
  failed to stat /var/run/xenstore.pid: No such file or directory
  cannot init xl context

The reason for the failure is that the NetBSD startup script
creates the file
  /var/run/xenstored.pid
but libxl checks for
  /var/run/xenstore.pid

Attached patch makes this consistent across Linux and NetBSD by
changing the Linux name (to the more-conventional food.pid form).

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>