libvirt: host specific virtio-rng backend

Users will be able to select an alternative source of entropy on the host,
by providing a path to the alternative device in nova.conf, instead of the
default /dev/random:

    rng_dev_path=/dev/hwrng

DocImpact
Partially implements blueprint libvirt-rng-device
Change-Id: Ibc2d1e9839ae2f5aeca14fbe966e8bc7c4427714
Merge "libvirt: host specific virtio-rng backend"