Loading Xen 4.8.1.config...
error: no multiboot header found.
Loading Linux 4.9.35.19.pvops.qubes.x86_64...
error: you need to load the kernel first.
Loading initial ramdisk...
error: you need to load the kernel first.
In the bootloader try to use a differnet kernel (when you get there, advanced options, Xen and choose another kernel)