Fix build of openvz on RHEL-5.

Fix build of openvz on RHEL-5.

When building libvirt on RHEL-5, I saw this error:

cc1: warnings being treated as errors
openvz/openvz_conf.c: In function 'openvzGetVPSUUID':
openvz/openvz_conf.c:835: warning: 'saveptr' may be used uninitialized in this function
make[3]: *** [libvirt_driver_openvz_la-openvz_conf.lo] Error 1

gcc in RHEL-5 gets upset about this usage of strtok_r (even though
it is perfectly valid).  Just set *saveptr to NULL at the
start to quiet it down.

Signed-off-by: Chris Lalancette <clalance@redhat.com>