VirtIO-RNG: Update default entropy source to `/dev/urandom`

When QEMU exposes a VirtIO-RNG device to the guest, that device needs a
source of entropy, and that source needs to be "non-blocking", like
`/dev/urandom`. However, currently QEMU defaults to the problematic
`/dev/random`, which on Linux is "blocking" (as in, it waits until
sufficient entropy is available).

Why prefer `/dev/urandom` over `/dev/random`?
---------------------------------------------

The man pages of urandom(4) and random(4) state:

"The /dev/random device is a legacy interface which dates back to a
time where the cryptographic primitives used in the implementation
of /dev/urandom were not widely trusted. It will return random
bytes only within the estimated number of bits of fresh noise in the
entropy pool, blocking if necessary. /dev/random is suitable for
applications that need high quality randomness, and can afford
indeterminate delays."

Further, the "Usage" section of the said man pages state:

"The /dev/random interface is considered a legacy interface, and
/dev/urandom is preferred and sufficient in all use cases, with the
exception of applications which require randomness during early boot
time; for these applications, getrandom(2) must be used instead,
because it will block until the entropy pool is initialized.

"If a seed file is saved across reboots as recommended below (all
major Linux distributions have done this since 2000 at least), the
output is cryptographically secure against attackers without local
root access as soon as it is reloaded in the boot sequence, and
perfectly adequate for network encryption session keys. Since reads
from /dev/random may block, users will usually want to open it in
nonblocking mode (or perform a read with timeout), and provide some
sort of user notification if the desired entropy is not immediately
available."

And refer to random(7) for a comparison of `/dev/random` and
`/dev/urandom`.

What about other OSes?
----------------------

`/dev/urandom` exists and works on OS-X, FreeBSD, DragonFlyBSD, NetBSD
and OpenBSD, which cover all the non-Linux platforms we explicitly
support, aside from Windows.

On Windows `/dev/random` doesn't work either so we don't regress.
This is actually another argument in favour of using the newly
proposed 'rng-builtin' backend by default, as that will work on
Windows.

- - -

Given the above, change the entropy source for VirtIO-RNG device to
`/dev/urandom`.

Related discussion in these[1][2] past threads.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html
-- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?"
[2] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html
-- "[RFC] Virtio RNG: Consider changing the default entropy source to
/dev/urandom"

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20190529143106.11789-2-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>