qga: drop unnecessary GA_CHANNEL_UNIX_LISTEN checks

Throughout the code there are c->listen_channel checks which manage the
listen socket file descriptor (waiting for accept(2), closing the file
descriptor, etc).  These checks are currently preceded by explicit
c->method == GA_CHANNEL_UNIX_LISTEN checks.

Explicit GA_CHANNEL_UNIX_LISTEN checks are not necessary since serial
channel types do not create the listen channel (c->listen_channel).

As more listen channel types are added, explicitly checking all of them
becomes messy.  Rely on c->listen_channel to determine whether or not a
listen socket file descriptor is used.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
