grub: the one bootloader to rule them all

This uses our new GRUB2 features to handle GPT priority partition
selection, terminal selection, OEM tweaks, etc. The old SYSLINUX and
PV-GRUB configs are now unused except for maintaining compatibility
with older installs. Of the old configs only the ones that
coreos-postinst copies are needed. The new setup supports using GRUB2
under Xen, giving us automatic fallback support on all of our platforms
for the very first time!

Since grub.cfg is copied into place instead of generated, build_image's
--boot_args option is no longer supported. It could be re-added later
with some sed goo but for now it is easy enough to just edit grub.cfg.
vm_image_util: add qemu_xen image type for testing

To aid testing things under Xen it helps to have a machine locally that
actually runs Xen! This isn't a particularly great setup but it works
well enough to simplify my own testing. Must be used with a developer
image and packages built with `USE=vm-testing` set to include the Xen
userspace tools.
vm_image_util: remove kexec/boot_kernel configuration

No longer applicable with the new bootloader setup.
Merge pull request #359 from marineam/grub

grub: the one bootloader to rule them all