vl: plug -object back into -readconfig

Commit bc2f4fcb ("qom: move user_creatable_add_opts logic to vl.c
and QAPIfy it", 2021-03-19) switched the creation of objects from
qemu_opts_foreach to a bespoke QTAILQ in preparation for supporting JSON
syntax in -object.

Unfortunately in doing so it lost support for [object] stanzas in
configuration files and also for "-set object.ID.KEY=VAL".  The latter
is hard to re-establish and probably best solved by deprecating -set.
This patch uses the infrastructure introduced by the previous two
patches in order to parse QOM objects correctly from configuration
files.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210524105752.3318299-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>