vl.c: -object: don't ignore duplicate 'id'

vl.c: -object: don't ignore duplicate 'id'

object_property_add_child() may fail if 'id' matches
an already existing object. Which means an incorrect
command line.
So instead of silently ignoring error, report it and
terminate QEMU.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>