Fix init_host checking moved instances

Commit 6fc00d3465185 (https://review.openstack.org/#/c/18486/)
introduced a couple of problems.

1) It introduced a method to try to deduce instance integer id from
   a name the driver reports and using instance_name_template.  This
   fails badly when you're using something like 'instance-%(uuid)s' for
   instance_name_template.  It's only used when the driver does not
   support the 'list_instance_uuids' method.. but XenAPI happens to
   not implement this method.

2) There's a call to _get_instance_volume_block_device_info() that is
   passing an instance_uuid when the method wants an instance.

Fixes bug 1097987 (#1 above) by doing a brute force match against
   all instances' names.
Fixes bug 1097978 (#2 above)

Change-Id: I2da7658dd906fa06f0e867a2b22bf14e7f34cb12