tests: work with older dbus

tests: work with older dbus

On RHEL 5, with dbus 1.1.2, compilation failed with:

virsystemdmock.c: In function 'dbus_connection_send_with_reply_and_block':
virsystemdmock.c:68: warning: implicit declaration of function 'dbus_message_set_serial'

Fix this by instead bypassing all attempts to use a dbus serial.

* tests/virsystemdmock.c (dbus_message_set_reply_serial): Add new
override.
(dbus_connection_send_with_reply_and_block): No longer bother with
the serial.

Signed-off-by: Eric Blake <eblake@redhat.com>