fix #2197: ui: editing CDROM doesn't require VM.Config.Disk rights

fix #2197: ui: editing CDROM doesn't require VM.Config.Disk rights

A user which just has the default role PVEVMUser on a VM is able to
edit the CDROM, either to no media or if storage permissions are
available also to other ISOs, through the API. So reflect this again
for the WebUI. We could add a check for "VM.Config.CDROM", but that's
not really worth, if we keep in mind that the Ext.Caps state is
heuristically only.

fixes commit d35b5b2afbe9287a90129d861cf6fa27b91c1ed3

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>