quorum: Forbid adding children in blkverify mode

quorum: Forbid adding children in blkverify mode

The blkverify mode of Quorum only works when the number of children is
exactly two, so any attempt to add a new one must return an error.

quorum_del_child() on the other hand doesn't need any additional check
because decreasing the number of children would make it go under the
vote threshold.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>