hwmonio:: Add Interface base class and tests

Enable injecting hwmonio::HwmonIO mocks for testing.

Tested: Ran on quanta-q71l and saw all sensors exported to dbus as
expected with the expected values.

Change-Id: I35912bf2a733932d9e1e774ff53b0114ae16560b
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/fan_pwm.cpp b/fan_pwm.cpp
index cf89552..9b43b17 100644
--- a/fan_pwm.cpp
+++ b/fan_pwm.cpp
@@ -26,7 +26,7 @@
         std::string empty;
         //Write target out to sysfs
         try {
-            ioAccess.write(
+            ioAccess->write(
                 value,
                 type,
                 id,
@@ -45,7 +45,7 @@
                         WriteFailure::CALLOUT_DEVICE_PATH(devPath.c_str()));
 
             auto file = sysfs::make_sysfs_path(
-                    ioAccess.path(),
+                    ioAccess->path(),
                     type,
                     id,
                     empty);