Add slots of channel and Add get_dev_pts_max_id()  for hotplug serial test 

1.libvirt_xml.devices: Add slots for channel.
    The latest slots includes 'alias' and 'address'.

2.  virttest.utils_misc: add function to get the maxi ID for pty chardev
        when hot-pluging in pseudoterminal interfaces to VM for pty device,
    we must specify the device source path (/dev/pts/<number>) for it.
    The function get_dev_pts_max_id() can used to get the maxi and
    occupied device number.
    and then, user can specify the available source path based on it.



Could you add your example into the comment of this function? When another one read this function again, it will help a lot for understanding.