cfg2cmd: fix serial-bus for spice foldersharing

cfg2cmd: fix serial-bus for spice foldersharing

Thanks to Gilberto Nunes for finding a bug where the VM would not start
with foldersharing enabled and the qemu agent option disabled [0].

The cause was that the device org.spice-space.webdav.0 would not find a
virtio-serial-bus in this situation.

Since we always create a virtio-serial-bus for the spice vdagent it
seems sensible to use that also for the foldersharing device by moving
it in front of the other spice devices.

[0]: https://pve.proxmox.com/pipermail/pve-devel/2019-October/039441.html

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>