Remove explicit guestfs=10.0.2.4:6666 kernel command line parameter.

Remove explicit guestfs=10.0.2.4:6666 kernel command line parameter.

Since we control the appliance tightly, we can just specify
that it will always use a particular host and port, and we
don't need to pass it on the command line each time.

Also the VMCHANNEL_* constants are only relevant to the
particular guestfwd vmchannel implementation, so we rename
them as GUESTFWD_*.