Documentation: kunit: document support for QEMU in kunit_tool

Document QEMU support, what it does, and how to use it in kunit_tool.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>