domain_addr.c: Fix virtio console port autoassign on virtio-serial bus

domain_addr.c: Fix virtio console port autoassign on virtio-serial bus

This change fixes an issue with virtio console port assignment on virtio-serial buses.
Currently, when trying to autoassign a virtio console device, the device cannot be
assigned to a port greater than 0 on virtio-serial buses.
You will receive the following error:

`virtio-serial-bus: A port already exists at id 0`

Therefore, the data needs to be passed back into info when allowZero is true.
We should also preserve the controller data when allowZero is true, and
propagate allowZero into virDomainVirtioSerialAddrNextFromController
to get an appropriate startPort.

Fixes: 16db8d2e ("Add functions to track virtio-serial addresses")
Signed-off-by: Aaron M. Brown <aaronmbr@linux.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>