OvmfPkg/SmbiosPlatformDxe: split GetXenSmbiosTables() decl. to new header

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>
Message-Id: <20210526201446.12554-40-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>