core: set floppy path after vds is set on run vm

As a preliminary step in making IsoPrefixVDSCommand a VdsBrokerCommand
instead of IrsBrokerCommand, the floppy path setting on run VM is moved
to be made after the destination host on which the VM will be run on is
set, so that we'll be able to query the iso prefix (which is needed for
floppy path setting) from that host instead of from the SPM.

Change-Id: I4737a3bc19e4810489eaf13a3ae98c07bfdd033e
Signed-off-by: Arik Hadas <ahadas@redhat.com>