e1000: allow command-line selection of card model

e1000: allow command-line selection of card model

Allow selection of different card models from the qemu
command line, to better accomodate a wider range of guests.

Signed-off-by: Romain Dolbeau <romain@dolbeau.org>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>