oem: add grub configs for gce and onmetal

Our GRUB setup now offers a proper hook for OEMs, these customizations
were previously handled by running sed on the old SYSLINUX configs.
xen: update to latest release, 4.4.1

I've seen somewhat more success running this version under QEMU than the
last time I tried. With luck I can get something all the way working. :)
coreos-dev: prune i7z from dev images, we don't need it

Left over from something chromeos.
coreos-dev: Add vm-testing use flag to pull in QEMU and Xen

Experimenting with ways to do more comprehensive platform testing.
If it proves practical then we can enable this by default.
coreos-kernel: enable Xen backend drivers and watchdog

For running CoreOS as a Dom0 host.
oem-xendom0: experimental OEM for testing Xen under QEMU

For use with the `qemu_xen` vm type. The base image's packages must be
built with the `USE=vm-testing` flag set in order to include Xen.
Merge pull request #974 from marineam/grub

Lots of bits for future GRUB and my own Xen testing