Now that rhbz#1014039 is fixed, GET https://rhevm34.example.com//api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000;current returns the current ISO image inserted in the CDROM, which can be different from the one which will be present at boot time (getting the same URL without ';current' would give us that image.
However, fetching the cdroms collection also gives the ISOs inserted in the cdrom, but ;current does not change anything there, the boot time ISO is always listed:

GET https://rhevm34.example.com//api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000;current

<cdrom href="/api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000" id="00000000-0000-0000-0000-000000000000"><vm href="/api/vms/$GUID" id="$GUID"/><file id="current.iso"/></cdrom>


GET https://rhevm34.example.com//api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000

<cdrom href="/api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000" id="00000000-0000-0000-0000-000000000000"><vm href="/api/vms/$GUID" id="$GUID"/><file id="boot.iso"/></cdrom>



GET https://rhevm34.example.com//api/vms/$GUID/cdroms;current

<cdroms>
  <cdrom href="/api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000" id="00000000-0000-0000-0000-000000000000"><vm href="/api/vms/$GUID" id="$GUID"/><file id="boot.iso"/></cdrom>
</cdroms>


GET https://rhevm34.example.com//api/vms/$GUID/cdroms

<cdroms>
  <cdrom href="/api/vms/$GUID/cdroms/00000000-0000-0000-0000-000000000000" id="00000000-0000-0000-0000-000000000000"><vm href="/api/vms/$GUID" id="$GUID"/><file id="boot.iso"/></cdrom>
</cdroms>


I'd expect to get current.iso in the 3rd GET.