plugins: move the more involved plugins to contrib

We have an exploding complexity problem in the testing so lets just
move the more involved plugins into contrib. tests/plugins still exist
for the basic plugins that exercise the API. We restore the old
pre-meson style Makefile for contrib as it also doubles as a guide for
out-of-tree plugin builds.

While we are at it add some examples to the documentation and a
specific plugins build target.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200909112742.25730-11-alex.bennee@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 7d0a5e9..018c4f9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2745,7 +2745,8 @@
 S: Maintained
 F: docs/devel/tcg-plugins.rst
 F: plugins/
-F: tests/plugin
+F: tests/plugin/
+F: contrib/plugins/
 
 AArch64 TCG target
 M: Richard Henderson <richard.henderson@linaro.org>