sysutils: add -x to cp in linktree

Since you can't make cross-device links, you'd think that cp -l (make
links) would imply -x (don't cross devices).

You'd be wrong.

src/pylorax/sysutils.py

@@ -106,5 +106,4 @@ def remove(target):
         os.unlink(target)
 
 def linktree(src, dst):
-    runcmd(["/bin/cp", "-al", src, dst])
-
+    runcmd(["/bin/cp", "-alx", src, dst])