devices: handle partscan loop devices

Treat loop device created with 'losetup -P' as regular
partitioned device - so if it has partition table,
prevent its usage in commands like 'pvcreate'.

Before 'pvcreate /dev/loop0' could have erased and formated as PV,
after this patch, device is filtered out and cannot be used.

lib/device/dev-type.h

@@ -43,6 +43,7 @@ struct dev_types {
 	int emcpower_major;
 	int power2_major;
 	int dasd_major;
+	int loop_major;
 	struct dev_type_def dev_type_array[NUMBER_OF_MAJORS];
 };