Change the Xen PVM to use the new kernel options

This patch changes the hypervisor to use the new kernel and initrd
instance attributes. They kernel is handled as expected (use the custom
or the default one), but the initrd is treated a little bit special, in
order to keep compatibility with previous behaviour:
  - if customized and not found, raise an error
  - if default and not found, do not use one

Reviewed-by: imsnah