rng: add request queue support to rng-random

rng: add request queue support to rng-random

Requests are now created in the RngBackend parent class and the
code path is shared by both rng-egd and rng-random.

This commit fixes the rng-random implementation which processed
only one request at a time and simply discarded all but the most
recent one. In the guest this manifested as delayed completion
of reads from virtio-rng, i.e. a read was completed only after
another read was issued.

By switching rng-random to use the same request queue as rng-egd,
the unsafe stack-based allocation of the entropy buffer is
eliminated and replaced with g_malloc.

Signed-off-by: Ladi Prosek <[email protected]>
Reviewed-by: Amit Shah <[email protected]>
Message-Id: <1456994238[email protected]>
Signed-off-by: Amit Shah <[email protected]>