e1000: fix compile warning introduced by security fix, and debugging

e1000: fix compile warning introduced by security fix, and debugging

e33f918c19e393900b95a2bb6b10668dfe96a8f2, the fix for XSA-41,
and its cherry picks in 4.2 and 4.1 introduced this compiler warning:
  hw/e1000.c:641: warning: 'return' with a value, in function returning void

In upstream qemu (where this change came from), e1000_receive returns
a value used by queueing machinery to decide whether to try
resubmitting the packet later.  Returning "size" means that the packet
has been dealt with and should not be retried.

In this old branch (aka qemu-xen-traditional), this machinery is
absent and e1000_receive returns void.  Fix the return statement.

Also add a debugging statement along the lines of the others in this
function.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>