core: set cd-rom path after vds is set on run vm

As a preliminary step in making IsoPrefixVDSCommand a VdsBrokerCommand
instead of IrsBrokerCommand, the cd-rom 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
cd-rom path setting) from that host instead of from the SPM.

This patch also changes the way the ISO path is determined: instead of
going over the possible values from the one with the lowest priotity to
the one with the highest priority and override the previous value each
time we find a valid value, we'll go over the possible values in the
reverse order and when finding a valid value we'll set the ISO path to
it and stop - it simplifies the code and it is more efficient that way.

Change-Id: Ic7919e224c093d86a2b1ebf564f0307d5dd813a7
Signed-off-by: Arik Hadas <ahadas@redhat.com>