fuzz: virtio: Add fuzzer for vring

Add a fuzzer to test the vring handling code against unexpected
mutations from the virtio device.

After building the sandbox with CONFIG_FUZZ=y, the fuzzer can be invoked
with by:

   UBOOT_SB_FUZZ_TEST=fuzz_vring ./u-boot

This fuzzer finds unvalidated inputs in the vring driver that allow a
buggy or malicious device to make the driver chase wild pointers.

Signed-off-by: Andrew Scull <ascull@google.com>