fish: Don't fail if -m and --listen flags are both given (RHBZ#612178).

fish: Don't fail if -m and --listen flags are both given (RHBZ#612178).

Testing this against a Fedora disk image:

$ ./fish/guestfish --ro -a F13.img -m /dev/sda1 --listen
export GUESTFISH_PID=6033
$ ./fish/guestfish --remote=6033 -- ping-daemon
$ ./fish/guestfish --remote=6033 -- ping-daemon
$ ./fish/guestfish --remote=6033 -- exit

Without this fix the first remote command would fail because
qemu would have already been killed.