1. The name parameter in below command must be org.libguestfs.channel.0. or else the guest would produce error 'failed to connect to virtio-serial' and '/dev/virtio-ports/org.libguestfs.channel.0: No such file or directory' even though there did exists a file 'com.test.my' in 'dev/virtio-ports' directory.

sudo kvm -drive file=ubuntu-1204-vm.img -m 512 -vga std -k en-us -vnc :1 -net nic -net tap,ifname=tap1,script=./qemu-ifup,downscript=no -daemonize -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x6 -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/test.agent,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.test.my

2. I leaved the first problem by altering the device name to default org.libguestfs.channel.0, now I can visit the live guest by guestfish. However, any write or edit operation like vi or write don't work even I set --rw option with guestfish. It seems like guestfish just write the results to a tmp file in /tmp/guestfish**/ directory rather than the guest image file. I don't know if it needs a writeback operation to make the changes happen. Besides, read operation like cat can only read files produced before the mount operation, any real time modification by the guest can't be found with guestfish --live. 

Any answers will be appreciated.