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_speed.cpp b/fan_speed.cpp
index f1afd55..9e297bc 100644
--- a/fan_speed.cpp
+++ b/fan_speed.cpp
@@ -21,7 +21,7 @@
         //Write target out to sysfs
         try
         {
-            ioAccess.write(
+            ioAccess->write(
                     value,
                     type,
                     id,
@@ -41,7 +41,7 @@
                         WriteFailure::CALLOUT_DEVICE_PATH(devPath.c_str()));
 
             auto file = sysfs::make_sysfs_path(
-                    ioAccess.path(),
+                    ioAccess->path(),
                     type,
                     id,
                     entry::target);
@@ -66,7 +66,7 @@
 
         try
         {
-            ioAccess.write(
+            ioAccess->write(
                     val,
                     type::pwm,
                     id,
@@ -85,7 +85,7 @@
                         WriteFailure::CALLOUT_DEVICE_PATH(devPath.c_str()));
 
             auto fullPath = sysfs::make_sysfs_path(
-                    ioAccess.path(),
+                    ioAccess->path(),
                     type::pwm,
                     id,
                     entry::enable);