xen/pvh: Fix compile issues with xen_pvh_domain()

Oddly enough it compiles for my ancient compiler but with
the supplied .config it does blow up. Fix is easy enough.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>