rng-random: add an RNG backend that uses /dev/random (v3)

rng-random: add an RNG backend that uses /dev/random (v3)

The filename can be overridden but it expects a non-blocking source of entropy.
A typical invocation would be:

qemu -object rng-random,id=rng0 -device virtio-rng-pci,rng=rng0

This can also be used with /dev/urandom by using the command line:

qemu -object rng-random,filename=/dev/urandom,id=rng0 \
     -device virtio-rng-pci,rng=rng0

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
v1 -> v2
 - merged header split patch into this one
v2 -> v3
 - bug fix in rng-random (Paolo)