virtio-serial: search for duplicate port names before adding new ports

Before adding new ports to VirtIOSerial devices, check if there's a
conflict in the 'name' parameter.  This ensures two virtserialports with
identical names are not initialized.

Reported-by: <mazhang@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
