eth: Extend vlan stripping functions

Make VLAN stripping functions return number of bytes
copied to given Ethernet header buffer.

This information should be used to re-compose
packet IOV after VLAN stripping.

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