This aligns the Xenserver plugin to being a bit more inline with what
Packer > 1.6.0 is expecting, since packer now simply ignores the
iso_checksum_type (it's supposed to error out but that code path isn't
working right now because we don't set PluginType in the configs. The
unit tests have been altered to reflect this reality.
Note that this isn't a comprehensive change; the config still has the
inert ISOChecksumType, and there's probably a laundry list of other
things that needs to be looked at, For now though, we have working
unit tests again.
Documentation from SDK has been aligned for iso_checksum
