add 3 bwlimit tests

1) checking that the empty storage list is treated correctly (only override
and datacenter config limit considered)
2) checking that the empty storage list is treated correctly (as with 1).
3) checking that undef can be passed as one element of the storage list (it is
ignored)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>