Allow to unshelve instance booted from volume

When we shelve an instance, volumes are not detached from cinder to kept
them 'In-Use' in cinder
But when you unshelve this instance, nova ask cinder the reattach theses
volumes. This fails, because volume cannot be attached twice.

This patch permits when we ask the libvirt DriverBlockDevice to attach a
device to the instance to bypass the cinder attachement code that is not needed
when we unshelve an instance, because the cinder volume is kept 'In-use'
during the 'shelved' state.

Co-Authored-By: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>
Closes-bug: #1305399
Change-Id: I780a9407feeb48ecd3e295508ce3e6bc3b09d3e6
Merge "Allow to unshelve instance booted from volume"