While receiving packets via e1000e_write_packet_to_guest() routine,
'desc_offset' is advanced only when RX descriptor is processed. And
RX descriptor is not processed if it has NULL buffer address.
This may lead to an infinite loop condition. Increament 'desc_offset'
to process next descriptor in the ring to avoid infinite loop.

Reported-by: Cheol-woo Myung <330cjfdn@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>