Handle kernel options for Grub 2 when using Xen.

When you are using the Xen virtualization hypervisor, you need to
set a different variable, GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT, in
/etc/default/grub to pass kernel options to the Dom0 Linux kernel,
instead of GRUB_CMDLINE_LINUX when not using Xen.  Detect when Xen
Dom0 is used in bootloader::grub2::kernel_option and do the right
thing.