the option for vdagent communication needed for qxl scren resizing is not documented

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1452742
Reported by 'Michal Suchanek' on 2015-05-07 :

Hello,

I tried running a guest with vdagent which is supposed to resize the guest
screen to match client window size.

However, a special serial port needs to be created for the vdagent to
communicate with the client.

This patch adds a short note to the vga qxl option.