libvirt: adding a random number generator device to instances

In order for the virtio-rng device to be added to KVM instance,
hw_rng=virtio property should be set in Glance image metadata:

glance image-update \
       --property hw_rng_model=virtio \
       [image]

The cloud administrator will have the option to limit
or disable the use of this device by setting the following flavour
extra_specs:

hw_rng:allowed=True - Allow virtio-rng to be added to instances.
hw_rng:rate_bytes - The allowed amount of bytes for the the guest
                    to read from the host’s entropy per period.
hw_rng:rate_period - Sets the duration of a read period in seconds.

DocImpact
Partially implements blueprint libvirt-rng-device

Change-Id: I022e322a7c414e9d618e60c2879aee903732581d
Merge "libvirt: adding a random number generator device to instances"