OvmfPkg/SmbiosPlatformDxe: split Xen entry point from QEMU entry point
    
    Remove the SmbiosTablePublishEntry() function from "SmbiosPlatformDxe.c".
    "SmbiosPlatformDxe.c" becomes hypervisor-agnostic.
    
    Add SmbiosTablePublishEntry() back, simplified for QEMU, to the existent
    file "Qemu.c". The GetQemuSmbiosTables() function no longer needs to be
    declared in "SmbiosPlatformDxe.h"; "SmbiosPlatformDxe.h" becomes
    hypervisor-agnostic.
    
    Add SmbiosTablePublishEntry() back, renamed and simplified for Xen, to the
    new, arch-independent file "Xen.c". (The existent Xen-specific C files are
    arch-dependent.)
    
    Update both INF files; remove the dependencies that are now superfluous in
    each.
    
    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: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>