Flexible guestfs_vmchannel parameter for future appliances.

Flexible guestfs_vmchannel parameter for future appliances.

This reimplements parts of commit da0a4f8d1f6ddd302ceba028d87c6e009589e503
in a different, but compatible way.

We pass guestfs_vmchannel=tcp:<ip>:<port> on the command line.  This
is intended to be used as follows (now and in future versions):

  tcp:10.0.2.4:6666    for guestfwd vmchannel
  tcp:10.0.2.2:<port>  for future "no vmchannel" implementation
  /dev/vcon4           for future virtio-console vmchannel*

It also accepts the old-style guestfs=10.0.2.4:6666 parameter which
is sent by older libraries, and turns this transparently into the
correct format above.

If no guestfs_vmchannel is passed, then this defaults to the guestfwd
vmchannel which older libraries would expect.

* Maybe this last one should be dev:/dev/vcon4 or file:/dev/vcon4, but
we don't need to decide that now.