qemu: Implement support for EGD backend for virtio-rng

qemu: Implement support for EGD backend for virtio-rng

This patch adds a new capability bit QEMU_CAPS_OBJECT_RNG_EGD and code
to support the egd backend for the VirtIO RNG device.

The device is added by 3 qemu command line options:
-chardev socket,id=charrng0,host=1.2.3.4,port=1234 (communication
                                                    backend)
-object rng-egd,chardev=charrng0,id=rng0 (RNG protocol client)
-device virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x4 (the RNG device)