xen: simplify init_hvm_pv_info

init_hvm_pv_info is called only in PVonHVM context, move it into ifdef.
init_hvm_pv_info does not fail, make it a void function.
remove arguments from init_hvm_pv_info because they are not used by the
caller.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>