XenAPI: Fix volume detach

fixes bug 1101229

The code-cleanup and code-move changes from this patch:
https://review.openstack.org/#/c/19219/
introduced a problem: the volume_utils.find_sr_from_vbd method was
called after the vbd has already been destroyed. This change fixes the
issue by moving the sr discovery before the vbd destruction, and tests
the call order by using side effects.

Change-Id: Ide4f8ac810f98bb192909f5f0408affc940e7446