OvmfPkg/SmbiosPlatformDxe: split GetXenSmbiosTables() decl. to new header
    
    Move the declaration of the GetXenSmbiosTables() function to a new header
    file called "XenSmbiosPlatformDxe.h". (The only declaration that remains
    in "SmbiosPlatformDxe.h" for now is that of GetQemuSmbiosTables().)
    
    Modify the pattern in "Maintainers.txt" so that the new file be covered in
    the "OvmfPkg: Xen-related modules" section.
    
    This patch is best viewed with "git show --no-renames".
    
    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>