Ovmf/Xen: move XenBusDxe hypercall code to separate library

Ovmf/Xen: move XenBusDxe hypercall code to separate library

This moves all of the Xen hypercall code that was private to XenBusDxe
to a new library class XenHypercallLib. This will allow us to reimplement
it for ARM, and to export the Xen hypercall functionality to other parts
of the code, such as a Xen console SerialPortLib driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-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@16970 6f19259b-4bc3-4df7-8a09-765794883524