Only first 4 disks can be emulated as IDE disks by QEMU. Specifically,
CDROM must be one of those first 4 disks, otherwise it will be
ignored. This is especially important if one wants to boot the VM from
that CDROM.
Since xvdd normally is a kernel-related volume (boot image, modules) it
makes perfect sense to re-use it for CDROM. It is either set for kernel
volume (in which case, VM should boot from it and not the CDROM), or
(possibly bootable) CDROM.
This needs to be done in two places:
- BlockExtension for dynamic attach
- libvirt xen.xml - for before-boot attach
In theory the latter would be enough, but it would be quite confusing
that device will get different options depending on when it's attached
(in addition to whether the kernel is set - introduced here).
This all also means, xvdd not always is a "system disk". Adjust listing
connected disks accordingly.
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