Provide prebuilt grub-xen binaries for host use in a new grub-xen-host package.

Provide prebuilt grub-xen binaries for host use in a new grub-xen-host package.

These can be referenced via kernel= in guest cfg to boot a guest using grub2.

debian/grub-xen-host_grub.cfg is the same as the qemu-firmware variant,
although I'm not sure what the best content is so it's more of a placeholder
for now.

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v3: Use EXTRA_PACKAGES, drop unnecessary debian/grub-xen-host.dirs, use
    "normal" command/shell to interpret the grub.cfg which allows proper use of
    "if" etc., use proposed spec paths. http://lists.xen.org/archives/html/xen-devel/2014-08/msg02964.html
v2: Include a usable grub.cfg