Use a single libusb context

libusb on Windows is limited to only a single context. Trying to open
more than one can cause device enumerations to fail randomly.

libusb is thread-safe and we don't use the manual polling support (with
`poll()`) so this should be safe.

Source/Core/Common/LibusbContext.h

@@ -0,0 +1,15 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <memory>
+
+struct libusb_context;
+
+namespace LibusbContext
+{
+// libusb on Windows is limited to only a single context. Trying to open more
+// than one can cause issues with device enumerations.
+// libusb is thread-safe so this context can be safely used from different threads.
+std::shared_ptr<libusb_context> Get();
+}