net/i40e: change VF default ITR interval

net/i40e: change VF default ITR interval

Current default ITR interval I40E_QUEUE_ITR_INTERVAL_DEFAULT is
used by both DPDK PF and DPDK VF, but this ITR interval value
will cause lots of interrupts when using VF. This patch modifies
VF default ITR interval to reduce interrupts.
ITR interval can be reconfigured by RTE_LIBRTE_I40E_ITR_INTERVAL
if expecting low latency.

Signed-off-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>