Add virtio rng device support

Add full support for boot guest with virtio-rng devices (both host passthrough device and egd backend are supported).  Set rng-random backend device as default virtio-rng device when boot up a guest, and /dev/random will be set as default backend of rng-random device.

rng-random backend device's backend can be changed by param filename_passthrough (eg. filename_passthrough = /dev/hwrng). For egd backend device's  we can setup a egd server on localhost if setup_egd, and default tcp port is '8000'. You can also setup 'host' and 'port' params to connect a remote egd.pl server but you need to take care that setup_egd should be set to 'no'


Thanks,
Xu