Adding pcie-root-port devices via QMP apparently isn't possible

[Thomas Parrott <thomas.parrott@canonical.com>]

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Due to QEMU moving towards a QMP configuration mechanism and away from
config file support, the LXD team are currently in the process of migrating
to using QMP to add devices to VMs (so that we can support the use of QEMU
6.0).

Currently we are using the `-S` flag to freeze CPU at startup, then using
QMP to add NIC devices via the `device_add` command, and then using the
`cont` command to start the VM guest.

However we have found that it is apparently not possible to add a
pcie-root-port device via QMP.

Initially we tried using something like:

device_add
{"id":"qemu_pcie5","driver":"pcie-root-port","bus":"pcie.0","addr":"1.5","chassis":5,"multifunction":"on"}

Which was a straight conversion from the current config file we use.

However this gave the error:

Error: Bus 'pcie.0' does not support hotplugging

Then I found the `--preconfig` flag which sounded interesting, so we
removed the use of `-daemonize` which prevents the use of --preconfig` and
then tried adding the pcie-root-port devices in the preconfig stage.

But this resulted in the error:

The command 'device_add' isn't permitted in 'preconfig' state

So we wondered is this a bug, or if not, what is the correct way to be
adding pcie-root-ports going forward?

Thanks
Tom Parrott

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