slirp: Put forked exec into separate process group

Recent smb daemons tend to terminate themselves via a process group
SIGTERM. If the daemon is still in qemu's group by that time, qemu will
die as well. Avoid this by always pushing fork_exec processes into a
group of their own, not just (unused) type 2 execs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>