fix(vm_image_util): Skip kexec when booting on Xen HVM

This is buggy, instead we will need to handle Xen HVM similar to PV
except this requires SYSLINUX config tweaking instead of grub's menu.lst
Merge pull request #186 from marineam/hvm

fix(vm_image_util): Skip kexec when booting on Xen HVM
fix(build_image): The kernel is now in /usr/boot