core: optional parameter storageFormatType not sent to vdsm

storageFormatType is marked in jsonrpc schema as optional but it was not
sent when the value was provided.


Bug-Url: https://bugzilla.redhat.com/1139817
Change-Id: Ia09978bcd9228591735a15a8d4a827db9f1b3ef2
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>