core: change iso prefix command to be vds broker command

Change IsoPrefixVDSCommand to query the iso prefix from the VDS with
the given ID instead of from the SPM (all the storage pool information
can be received from hsm as well). That way, we'll be able to run vm
with floppy/cd-rom even when there is no SPM defined in the DC.

Change-Id: I272ce7b0407bf83bd47646941630362ecf0b18cc
Signed-off-by: Arik Hadas <ahadas@redhat.com>