Clean up when "gnt-instance modify" fails to create a disk

cmdlib.LUInstanceSetParams now uses helper functions to create and wipe
disks, so that when the creation of a disk fails, any leftover device is
cleaned up. As a bonus, exceptions raised by _CreateBlockDev() are caught
correctly.

Now cmdlib._CreateDisks() is used every time there are disks to create.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>