MAINTAINERS: Extend the device fuzzing section

The file docs/devel/fuzzing.txt should be in this section, too, and add
myself as a reviewer (since I often take the fuzzer patches through the
qtest-next tree, I should be notified on patches, too).

Message-Id: <20200721053926.17197-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>