Add missing paramter and unittests for this case

While git-am'ing the blockdev_prefix patch, I modified the hv_xen.py
file but forgot to amend the patch :(, so here it is.

Furthermore, to detect such inconsistencies in the future, a unittest is
added that cross-checks the default parameters in constants.py and the
hypervisor PARAMETERS dict.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>