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>