vm/guest: Remove rngd (obsoleted by khwrngd)

vm/guest: Remove rngd (obsoleted by khwrngd)

khwrngd provides the same functionality as rngd here, but is integrated in the
kernel, and started by default by the hwrng driver as soon as the device is
initialized.

It was added in Linux 3.17 by the following commit:

commit be4000bc4644d027c519b6361f5ae3bbfc52c347
Author: Torsten Duwe <duwe@lst.de>
Date:   Sat Jun 14 23:46:03 2014 -0400

    hwrng: create filler thread

    This can be viewed as the in-kernel equivalent of hwrngd;
    like FUSE it is a good thing to have a mechanism in user land,
    but for some reasons (simplicity, secrecy, integrity, speed)
    it may be better to have it in kernel space.

    This patch creates a thread once a hwrng registers, and uses
    the previously established add_hwgenerator_randomness() to feed
    its data to the input pool as long as needed. A derating factor
    is used to bias the entropy estimation and to disable this
    mechanism entirely when set to zero.