virtio-serial: Clean up virtconsole detection

virtio-serial-bus needs to treat "virtconsole" devices specially.  It
uses VirtIOSerialPort member is_console to recognize them.  It gets
its value via property initialization.  Cute hack, except it lets
users mess with it: "-device virtconsole,is_console=0" isn't plugged
into port 0 as it should.

Move the flag to VirtIOSerialPortInfo.  Keep the property for backward
compatibility; its value has no effect.

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