hw/arm/bcm2836: Fix crash with device_add bcm2837 on unsupported machines

hw/arm/bcm2836: Fix crash with device_add bcm2837 on unsupported machines

When trying to "device_add bcm2837" on a machine that is not suitable for
this device, you can quickly crash QEMU afterwards, e.g. with "info qtree":

echo "{'execute':'qmp_capabilities'} {'execute':'device_add', " \
 "'arguments':{'driver':'bcm2837'}} {'execute': 'human-monitor-command', " \
 "'arguments': {'command-line': 'info qtree'}}" | \
 aarch64-softmmu/qemu-system-aarch64 -M integratorcp,accel=qtest -S -qmp stdio

{"QMP": {"version": {"qemu": {"micro": 50, "minor": 12, "major": 2},
 "package": "build-all"}, "capabilities": []}}
{"return": {}}
{"error": {"class": "GenericError", "desc": "Device 'bcm2837' can not be
 hotplugged on this machine"}}
Segmentation fault (core dumped)

The qdev_set_parent_bus() from instance_init adds a link to the child devices
which is not valid anymore after the bcm2837 instance has been destroyed.
Unfortunately, the child devices do not get destroyed / unlinked correctly
because both object_initialize() and object_property_add_child() increase
the reference count of the child objects by one, but only one reference
is dropped when the parent gets removed. So let's use the new functions
object_initialize_child() and sysbus_init_child_obj() instead to create
the objects, which will take care of creating the child objects with the
correct reference count of one.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1531745974-17187-4-git-send-email-thuth@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>