libvirt: Fix typo in configdrive implementation

libvirt configdrive implementation
references 'instane' instead of 'instance'.
Also, add a test for adding configdrive block device
that would catch such an error

fixes bug 1101352

Change-Id: I36cba4487ad9d78d4e0366c8c2550603ba155c47
Merge "libvirt: Fix typo in configdrive implementation"