virtio-rng backend should use getentropy() syscall when available

According to https://wiki.qemu.org/Features/VirtIORNG the default backend for `virtio-rng-pci` is `/dev/random`.  Alternately, the user can point it to a different backend file, like `/dev/urandom`.

However, both of these files have suboptimal behavior in one way or another, as documented in `random(7)`.  Instead, the default behavior should be to pull the requested octets from the `getrandom()` system call, if available, called with no flags set.