virtio-serial: Plug memory leak on qdev exit()

virtio_serial_init() allocates the VirtIOSerialBus dynamically, but
virtio_serial_exit() doesn't free it.

Fix by getting rid of the allocation.

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