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.