Remove unused virt.disk.api methods bind/unbind

The bind and unbind methods were only used by the old LXC
attach/detach volume code which was removed in
https://review.openstack.org/17861 (commit
b8831091a8d097bb55a0e65c3af254a8bfa88811). Remove them.

Fixes bug 1156127

Change-Id: I37b0344970415fe93faabd955b4b4db88de269db
Merge "Remove unused virt.disk.api methods bind/unbind"