Missing rollback logic when image creating for volume based instance

[Alex Xu]

In the code of create image for volume based instance http://git.openstack.org/cgit/openstack/nova/tree/nova/compute/api.py#n2610, there isn't rollback code when create volume snaphost or create image failed. For example, both cinder and glance have quota for the number of volume snapshot and the number of image metadata, so there have a chance to failed on quota. But the code won't rollbacked already created volume snapshot.