migration: Make savevm.c target independent

It only needed TARGET_PAGE_SIZE/BITS/BITS_MIN values, so just export
them from exec.h

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/exec.c b/exec.c
index de2961b..ff16f04 100644
--- a/exec.c
+++ b/exec.c
@@ -3444,6 +3444,15 @@
     return TARGET_PAGE_SIZE;
 }
 
+int qemu_target_page_bits(void)
+{
+    return TARGET_PAGE_BITS;
+}
+
+int qemu_target_page_bits_min(void)
+{
+    return TARGET_PAGE_BITS_MIN;
+}
 #endif
 
 /*