OvmfPkg/XenBusDxe: replace MIT license text with SPDX ID

OvmfPkg/XenBusDxe: replace MIT license text with SPDX ID

The "XenBus.c", "XenStore.c", "XenStore.h" files under "OvmfPkg/XenBusDxe"
include the MIT License text. Replace those license blocks with
"SPDX-License-Identifier: MIT" (which refers to
<https://spdx.org/licenses/MIT.html>) for easier parsing by machines (and
humans).

This follows the tree-wide adoption of "SPDX-License-Identifier"s, made
for <https://bugzilla.tianocore.org/show_bug.cgi?id=1373>.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: xen-devel@lists.xenproject.org
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1654
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>