Return the number of disks present in the configuration of the test
domain when called with @errors as NULL and @maxerrors as 0.

Otherwise report an error for every second disk, assigning available
error codes in a cyclic order.
Signed-off-by: NIlias Stamatis <stamatis.iliass@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>