Enable new-style -chardev ... guestfwd command line.

Newer versions of qemu have changed the command line format
(again).  '-net channel' is now deprecated.  Instead we use:

    -chardev socket,id=guestfsvmc,path=/path/to/sock,server,nowait
    -net user,vlan=0,net=10.0.2.0/8,guestfwd=tcp:10.0.2.4:6666-chardev:guestfsvmc
    -net nic,model=ne2k_pci,vlan=0

The old style format is still used if the new style is not
detected at runtime.