net/i40e: fix clear xstats bug in VF

net/i40e: fix clear xstats bug in VF

Clearing VSI stats done by setting an offset value to latest stats
and further stats values are subtracted from that offset.

This setting offset and baselining logic exists in PF driver, but for VF
there is no way to update its VSI offset in the PF.

So same baselining logic added into VF, also setting VSI offset in VF
fixed to support VF stats clear.

Fixes: da61cd0849766 ("i40evf: add extended stats")
Cc: stable@dpdk.org
Signed-off-by: Wei Zhao <wei.zhao1@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>