Virtual machines don’t produce a lot of entropy on their own. Typically the need for additional entropy triggers talk of hardware based entropy generators or network based entropy distribution protocols. Sometimes you just need a little bit of entropy for a moment.
$ sbuild-update --keygen Generating archive key. Not enough random bytes available. Please do some other work to give the OS a chance to collect more entropy! (Need 279 more bytes)
Disk tends to be one of the only remaining sources of entropy on virtual systems. I usually do something like this:
$ while true ; do cat /proc/sys/kernel/random/entropy_avail ; \ sudo find / > /tmp/find.log ; sync ; done
This numbers printed should go up and down as your application consumes the entropy. Hit CTRL+C when you’ve got enough. This is probably a bad source of entropy, but the world is inherently dangerous.