lvextend the device rootfs 
change the boot device of the vm
restart the vm and connect to it (vnc or whatever)
fdisk on the device
delete the partition and recreate it fatter
write!
e2fsck -f /dev/wotwotXXX
resize2fs /dev/wotwotXXX newSize
destroy the vm
change the boot device AGAIN to hd
then start the VM and you're done