vdsm: add support for virtio-rng devices

This patch adds support for virtio-rng device, allowing
passthrough of /dev/random and /dev/hwrng. getVdsCapabilities
additionally returns rngSources, a list of host's available
entropy sources.

Change-Id: I4b9b2c355e06bae5f66bbaadffd5fda8d3ed4e9a
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19091
Reviewed-by: Dan Kenigsberg <danken@redhat.com>