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/Common.vcxproj

@@ -113,6 +113,7 @@
     <ClInclude Include="Hash.h" />
     <ClInclude Include="IniFile.h" />
     <ClInclude Include="JitRegister.h" />
+    <ClInclude Include="LibusbContext.h" />
     <ClInclude Include="LinearDiskCache.h" />
     <ClInclude Include="MathUtil.h" />
     <ClInclude Include="MD5.h" />
@@ -159,6 +160,9 @@
     <ClCompile Include="Hash.cpp" />
     <ClCompile Include="IniFile.cpp" />
     <ClCompile Include="JitRegister.cpp" />
+    <ClCompile Include="LibusbContext.cpp">
+      <DisableSpecificWarnings>4200;%(DisableSpecificWarnings)</DisableSpecificWarnings>
+    </ClCompile>
     <ClCompile Include="Logging\ConsoleListenerWin.cpp" />
     <ClCompile Include="MathUtil.cpp" />
     <ClCompile Include="MD5.cpp" />