pcnet: add loopback mode emulation

This patch enhances the pcnet NIC emulation with better loopback mode
support, including CRC generation for looped-back packets in "raw" mode.
The patch has practically no impact on the normal RX and TX path.

Successfully tested against an ancient proprietary pcnet driver that
does a lot of hardware checks on boot-up and now works fine over qemu as
well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5135 c046a42c-6fe2-441c-8c8c-71466251a162
