NetRxPkt: Account buffer with ETH header in IOV length

In case of VLAN stripping ETH header is stored in a
separate chunk and length of IOV should take this into
account.

This patch fixes checksum validation for RX packets
with VLAN header.

Devices affected by this problem: e1000e and vmxnet3.

Cc: qemu-stable@nongnu.org
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
