Ovmf/Xen: move Xen interface version to <xen.h>

Ovmf/Xen: move Xen interface version to <xen.h>

Tiancore has its private copy of the Xen headers, and all drivers
that depend on it should use the same Xen interface version, so
let's move the #define to xen.h itself.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16967 6f19259b-4bc3-4df7-8a09-765794883524