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>