[porthole] initial implementation of the porthole device interface

This is known as 'introspection' in the gnif/qemu repo, it's name is not
final, however porthole is more appropriate but also may not be the
final name.

Note: This branch is experiemental and may never be released if QEMU do
not accept the patch for the new device upstream.

porthole/src/linux/CMakeLists.txt

@@ -0,0 +1,13 @@
+cmake_minimum_required(VERSION 3.0)
+project(porthole-linux LANGUAGES C)
+
+add_library(porthole-linux STATIC
+	device.c
+)
+
+#target_link_libraries(porthole-linux)
+
+target_include_directories(porthole-linux
+	PRIVATE
+		src
+)