hw: do not pass NULL to memory_region_init from instance_init

hw: do not pass NULL to memory_region_init from instance_init

This causes the region to outlive the object, because it attaches the
region to /machine.  This is not nice for the "realize" method, but
much worse for "instance_init" because it can cause dangling pointers
after a simple object_new/object_unref pair.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1443689999-12182-3-git-send-email-armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>