quorum: Add quorum_getlength().

Check that every bs file returns the same length.
Otherwise, return -EIO to disable the quorum and
avoid length discrepancy.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
