for pkvm osimage, the netboot value should be petitboot

perl-xCAT/xCAT/Utils.pm

@@ -4594,6 +4594,8 @@ sub lookupNetboot{
        }elsif($osarch =~ /^ppc64$/i){
           if(($osv =~ /rh/i and $osn < 7) or ($osv =~ /sles/i and ($osn < 11 or ($osn == 11 and $osm < 4)))){
              $ret="yaboot";
+          }elsif($osv =~ /pkvm/i){
+             $ret="petitboot";
           }else{
              $ret="grub2,grub2-tftp,grub2-http";
           }