NetRxPkt: Do not try to pull more data than present

In case of VLAN stripping, ETH header put into a
separate buffer, therefore amont of data copied
from original IOV should be smaller.

Cc: qemu-stable@nongnu.org
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit d5e77214)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>