fuzz: add an instrumentation filter

fuzz: add an instrumentation filter

By default, -fsanitize=fuzzer instruments all code with coverage
information. However, this means that libfuzzer will track coverage over
hundreds of source files that are unrelated to virtual-devices. This
means that libfuzzer will optimize inputs for coverage observed in timer
code, memory APIs etc. This slows down the fuzzer and stores many inputs
that are not relevant to the actual virtual-devices.

With this change, clang versions that support the
"-fsanitize-coverage-allowlist" will only instrument a subset of the
compiled code, that is directly related to virtual-devices.

Signed-off-by: Alexander Bulekov <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>