Ovmf/Xen: fix pointer to int cast in XenBusDxe

Ovmf/Xen: fix pointer to int cast in XenBusDxe

On ARM, xen_pfn_t is 64 bits but the size of a pointer is only
32 bits, so casting between them needs to go via (UINTN). Also
move the xen_pfn_t cast outside the shift so that we can avoid
shifting 64-bit quantities on 32-bit architectures, which may
require runtime library support.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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@16968 6f19259b-4bc3-4df7-8a09-765794883524