OvmfPkg/XenAcpiPlatformDxe: create from AcpiPlatformDxe

OvmfPkg/XenAcpiPlatformDxe: create from AcpiPlatformDxe

Create an almost verbatim copy of the
"OvmfPkg/AcpiPlatformDxe/AcpiPlatformDxe.inf" driver for the OvmfXen
platform. We're going to trim the driver in subsequent patches.
Ultimately, the XenAcpiPlatformDxe driver will duplicate a negligible
amount of code that is currently present in the QemuFwCfgAcpiPlatformDxe
driver.

List the new driver in "Maintainers.txt", in the "OvmfPkg: Xen-related
modules" section.

Switch the OvmfXen platform to the new driver at once.

This patch should be reviewed with "git show --find-copies-harder".

Cc: Andrew Fish <afish@apple.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-12-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>