pcnet: switch to use qemu_receive_packet() for loopback

pcnet: switch to use qemu_receive_packet() for loopback

Git-commit: 99ccfaa1edafd79f7a3a0ff7b58ae4da7c514928

This patch switches to use qemu_receive_packet() which can detect
reentrancy and return early.

This is intended to address CVE-2021-3416.

Cc: Prasad J Pandit <ppandit@redhat.com>
Cc: qemu-stable@nongnu.org
Buglink: https://bugs.launchpad.net/qemu/+bug/1917085
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Bruce Rogers <brogers@suse.com>