testing: Use host's /dev/urandom as /dev/random on guests via VirtIO RNG

testing: Use host's /dev/urandom as /dev/random on guests via VirtIO RNG

Newer versions of systemd etc. seem to require quite a lot of entropy
from /dev/random while booting, which can block and therefore delay the
start of other services (in particular sshd) by more than a minute.
Using the host's /dev/urandom via VirtIO RNG, we can avoid blocking the
guests.

The required kernel options are added for kernel versions 5.4+.