net_util: Replace use of rand crate with libc::getrandom()

The rand crate provides a full cross platform true cryptographic random
number implementation. As such it brings it lots of othe dependencies
and increases our binary size and compile time. This is excessive for
generating a MAC address.

From the cargo tree output:

│   │   ├── rand v0.8.3
│   │   │   ├── libc v0.2.86
│   │   │   ├── rand_chacha v0.3.0
│   │   │   │   ├── ppv-lite86 v0.2.10
│   │   │   │   └── rand_core v0.6.0
│   │   │   │       └── getrandom v0.2.0
│   │   │   │           ├── cfg-if v0.1.10
│   │   │   │           └── libc v0.2.86
│   │   │   └── rand_core v0.6.0 (*)

And cargo bloat:

 0.0%   0.4% 40.4KiB rand_chacha rand_chacha::guts::refill_wide::impl_sse2
 0.0%   0.4% 40.0KiB rand_chacha rand_chacha::guts::refill_wide::impl_ssse3
 0.0%   0.3% 37.6KiB rand_chacha rand_chacha::guts::refill_wide::impl_avx
 0.0%   0.3% 37.2KiB rand_chacha rand_chacha::guts::refill_wide::impl_sse41
 0.0%   0.2% 26.1KiB rand_chacha rand_chacha::guts::refill_wide::impl_avx2

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

Cargo.lock

@@ -702,7 +702,6 @@ dependencies = [
  "log 0.4.14",
  "net_gen",
  "pnet",
- "rand 0.8.3",
  "serde",
  "serde_json",
  "virtio-bindings",