Remove XEN_CMD from constants, adjust to PowercycleNode

Remove XEN_CMD from constants, adjust to PowercycleNode

This patch extend the RPC to powercycle a node by a
hvparams parameter. This parameter is used by the
xen hypervisor to determine which xen command to use.
The respective LU is adjusted as well.

Since this was the last operation depending on the
constants.XEN_CMD, it is removed eventually.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>