vm-virtio: Add virtio-rng implementation

Most of the code is taken from crosvm(bbd24c5) but is modified to
be adapted to the current VirtioDevice definition and epoll
implementation.

A new command option '--rng' is provided and it gives one the option
to override the entropy source which is /dev/urandom by default.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>

vm-virtio/src/lib.rs

@@ -24,6 +24,7 @@ mod block;
 mod device;
 pub mod net;
 mod queue;
+mod rng;
 
 pub mod transport;
 
@@ -31,6 +32,7 @@ pub use self::block::*;
 pub use self::device::*;
 pub use self::net::*;
 pub use self::queue::*;
+pub use self::rng::*;
 
 const DEVICE_INIT: u32 = 0x00;
 const DEVICE_ACKNOWLEDGE: u32 = 0x01;