This extends the permission bits of op blocker API to external using
Linux OFD locks.

Each permission in @perm and @shared_perm is represented by a locked
byte in the image file.  Requesting a permission in @perm is translated
to a shared lock of the corresponding byte; rejecting to share the same
permission is translated to a shared lock of a separate byte. With that,
we use 2x number of bytes of distinct permission types.

virtlockd in libvirt locks the first byte, so we do locking from a
higher offset.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>