virrandom: Make virRandomBits better

virrandom: Make virRandomBits better

Now that we have strong PRNG generator implemented in
virRandomBytes() let's use that instead of gnulib's random_r.

Problem with the latter is in way we seed it: current UNIX time
and libvirtd's PID are not that random as one might think.
Imagine two hosts booting at the same time. There's a fair chance
that those hosts spawn libvirtds at the same time and with the
same PID. This will result in both daemons generating the same
sequence of say MAC addresses [1].

1: https://www.redhat.com/archives/libvirt-users/2018-May/msg00097.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>