This patch makes three functions report whether the necessary permission
change tightens restrictions or not.  These functions are:
- bdrv_check_perm()
- bdrv_check_update_perm()
- bdrv_child_check_perm()

Callers can use this result to decide whether a failure is fatal or not
(see the next patch).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>