quorum: Fix child permissions

quorum: Fix child permissions

Quorum cannot share WRITE or RESIZE on its children.  Presumably, it
only does so because as a filter, it seemed intuitively correct to point
its .bdrv_child_perm to bdrv_filter_default_perm().

However, it is not really a filter, and bdrv_filter_default_perm() does
not work for it, so we have to provide a custom .bdrv_child_perm
implementation.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200218103454.296704-6-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>