xenapi: make boot from volume use volumeops

Currently the boot from volume code path has partially duplicated code
with the connect_volume method in volumeops and get_vdi_uuid_for_volume
in vm_utils.

This change removes the code duplication by calling volumeops to connect
the volume when doing boot from volume. For dependency reasons, this
means moving get_vdis_for_instance from vm_utils into vmops.

The unit test test_vdis_for_instance_bdi_password_scrubbed only checked
that auth_password was scrubbed if it was passed into the logging in a
certain way. The test now checks that auth_password is actually
passed into the logging in that way too.

Change-Id: I8591f30b177db38073a0145b902b607bb012a18a
Merge "xenapi: make boot from volume use volumeops"