storage/reflink: bail out early on most FICLONE errnos

Don't fall back on 'cp' if the FICLONE ioctl gives an errno that's not
plausibly reflink specific, because in such a case any fallback could
theoretically mask real but intermittent system/storage errors.

Looking through ioctl_ficlone(2) and the kernel source, it should be
sufficient to do the fallback only on EBADF/EINVAL/EOPNOTSUPP/EXDEV.
(EISDIR/ETXTBSY don't apply to this storage driver, which will never
legitimately attempt to reflink a directory or an active - in the
storage domain - swap file.)

qubes/storage/reflink.py

@@ -446,7 +446,10 @@ def _attempt_ficlone(src, dst):
     try:
         fcntl.ioctl(dst.fileno(), FICLONE, src.fileno())
         return True
-    except OSError:
+    except OSError as ex:
+        if ex.errno not in (errno.EBADF, errno.EINVAL,
+                            errno.EOPNOTSUPP, errno.EXDEV):
+            raise
         return False
 
 def _copy_file(src, dst):