qcow2: introduce check_refcounts_l1/l2() flags

The check_refcounts_l1/l2() functions have a check_copied argument to
check that the QCOW_O_COPIED flag is consistent with refcount == 1.
This should be a bool, not an int.

However, the next patch introduces qcow2 fragmentation statistics and
also needs to pass an option to check_refcounts_l1/l2().  This is a good
opportunity to use an int flags field.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>