e1000: clear EOP for multi-buffer descriptors

The e1000 spec says: if software statically allocates
buffers, and uses memory read to check for completed descriptors, it
simply has to zero the status byte in the descriptor to make it ready
for reuse by hardware. This is not a hardware requirement (moving the
hardware tail pointer is), but is necessary for performing an in–memory
scan.

Thus the guest does not have to clear the status byte.  In case it
doesn't we need to clear EOP for all descriptors
except the last.  While I don't know of any such guests,
it's probably a good idea to stick to the spec.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Juan Quintela <quintela@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>