references: http://sourceforge.net/p/smartmontools/mailman/message/32091488/

You can only use the smartmontools on devices that have an actual physical hard drive behind them. vd* devices are just virtual hard drives, that are emulated by the host operating system to provide storage to the guests. They are usually physical files on the hard drive of the host or a part of an lvm. The smartmontools use commands that only work on physical devices to get the smart stats. Virtual devices don't even have SMART or a physical ATA/SCSI interface attached to them, so it obviously can't work. Getting SMART data from a virtual device doesn't even make sense, because a virtual device itself can't file in a way SMART could detect it.