fuzz: adapt zimage fuzzer to new kernel loader API
fuzz: adapt zimage fuzzer to new kernel loader API

After CL:1636685, the kernel_loader API is different - we need to pass a
File instead of just a slice to load_kernel().

Borrow and adapt the make_elf_bin() function from the kernel_loader
tests to create a shared memory file from a slice of bytes to fix the
fuzzer.

BUG=chromium:970981
TEST=USE='asan fuzzer' emerge-nami crosvm

Change-Id: Ic17f6479fb355d45063ce6292552cb1e5664831a
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1645039
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm