OvmfPkg: SmbiosPlatformDxe: move IsEntryPointStructureValid() to Xen.c

OvmfPkg: SmbiosPlatformDxe: move IsEntryPointStructureValid() to Xen.c

This function is only called from Xen.c, so it should be defined in Xen.c
and have internal linkage (ie. STATIC).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Gabriel Somlo <somlo@cmu.edu>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18039 6f19259b-4bc3-4df7-8a09-765794883524