disk plugin: ignore loop devices

It doesn't make sense to apply tuning parameters to loop devices, since they
point at files that live in other block devices, also subject to tuning.

Signed-off-by: Andreas Hasenack <andreas.hasenack@canonical.com>

Author: panlinux

tuned/plugins/plugin_disk.py

@@ -132,9 +132,20 @@ def _is_hdparm_apm_supported(self, device):
 		self._hdparm_apm_device_support[device] = True
 		return True
 
+	@staticmethod
+	def _is_loop_device(device):
+		dev = device.attributes.get("dev", None)
+		if dev:
+			try:
+				major = int(dev.decode("utf-8").split(":")[0])
+			except:
+				major = None
+		return major == 7
+
 	@classmethod
 	def _device_is_supported(cls, device):
 		return  device.device_type == "disk" and \
+			not cls._is_loop_device(device) and \
 			device.attributes.get("removable", None) == b"0" and \
 			(device.parent is None or \
 					device.parent.subsystem in ["scsi", "virtio", "xen", "nvme"])