vl: fix -sandbox parsing crash when seccomp support is disabled

vl: fix -sandbox parsing crash when seccomp support is disabled

$ ./x86_64-softmmu/qemu-system-x86_64 -sandbox off
qemu-system-x86_64: -sandbox off: There is no option group 'sandbox'
Segmentation fault

Commit 5780760f5e ("seccomp: check TSYNC host capability") wrapped one
use of the sandbox option group to produce a sensible error, it didn't
do the same for another call to qemu_opts_parse_noisily():

(gdb) bt
    at util/qemu-option.c:829
 #0  0x00000000105b36d8 in opts_parse (list=0x0, params=0x3ffffffffab5 "off", permit_abbrev=true, defaults=false, errp=0x3ffffffff080)
     at util/qemu-option.c:829
 #1  0x00000000105b3b74 in qemu_opts_parse_noisily (list=<optimized out>, params=<optimized out>, permit_abbrev=<optimized out>) at util/qemu-option.c:890
 #2  0x0000000010024964 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:3589

Fixes: 5780760f5ea6163939a5dabe7427318b4f07d1a2
Cc: david@gibson.dropbear.id.au
Cc: otubo@redhat.com
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190429134757.13570-1-marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>