lib/event: update the check of poller returned status

Per the definition of the poller returned value
 * return 0 to indicate that polling took place but no events were found;
 * return positive to indicate that polling took place and some events were processed;
 * return negative if the poller does not provide spin-wait information.

Have a change to compare whether positive.

Change-Id: Ic01fe7a663afeccbb145979670234e023dac1d07
Signed-off-by: GangCao <gang.cao@intel.com>
Reviewed-on: https://review.gerrithub.io/428576


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>