Change screen resolution of Xenserver Console for a CentOS VM

I need to increase screen resolution to 1024x800 for my XEN console. I tried to place vga=791 at the end of kernel line the file /boot/grub/grub.conf but it seems that most of the boot arguments are ignored during startup. Probably inside XenSever /boot is not really used in order to launch CentOS. I even tried to add boot option in the boot option tab (VM -> Property -> Boot Option) but doesn't work.