libvirt_vm: remove setup_serial_ports() method.

Commit c37a86ca picked out the create_serial_console() from setup_serial_ports(),
and call it in env_process(). But it is not robust enough.

This patch rename the setup_serial_ports() to create_serial_console() to be called
in env_process() and remove the older create_serial_console().

Reported-by: Yanbing Du <ydu@redhat.com>
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>