Something that just caught me out, when associating a Xen VM with a foreman Host. The MAC comparison is case sensitive. I ended up with with the MAC in XenCenter recorded as "02:00:89:DE:AB:43" but the MAC recorded in Foreman as "02:00:89:de:ab:43", this caused the error 'No Host found to associate this vm with". Changing the MAC to be lower case is XenCentre solved the issue. It would be better if this comparison was case insensitive.