This backend talks EGD to a CharDriverState. A typical way to invoke this would
be:
qemu -chardev socket,host=localhost,port=1024,id=chr0 \
-object rng-egd,chardev=chr0,id=egd0 \
-device virtio-rng-pci,rng=egd0
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>