Add the iscsi device check and exception processing.

Before downloading the image and executing the command "qemu-img convert",
check if the iSCSI device is still available via the command "sudo dd
if=<device> of=/dev/null count=1". This command will raise an exception with
the message "Input/output error", if the back-end storage is disconnected to
the cinder-volume node, so we use it to test the availability of the storage
device. If it is unavailable, there is no need to download the image &
"qemu-img convert" and an exception DeviceUnavailable will be raised.

Fixed Bug1169290.

Change-Id: I133b4cc1bac493df073d42e240092cf2e6300454