savevm: unexport qemu_ftell()

It was unused out of savevm.c.

Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/savevm.c b/savevm.c
index ba69b44..32fe7a2 100644
--- a/savevm.c
+++ b/savevm.c
@@ -664,7 +664,7 @@
     return result;
 }
 
-int64_t qemu_ftell(QEMUFile *f)
+static int64_t qemu_ftell(QEMUFile *f)
 {
     return f->buf_offset - f->buf_size + f->buf_index;
 }