qdev: Don't assume existence of parent bus on unparenting

qdev: Don't assume existence of parent bus on unparenting

Commit 667d22d1ae59da46b4c1fbd094ca61145f19b8c3 (qdev: move bus removal
to object_unparent) made the assumption that at unparenting time
parent_bus is not NULL. This assumption is unjustified since
object_unparent() may well be called directly after object_initialize(),
without any qdev_set_parent_bus().

This did not cause any issues yet because qdev_[try_]create() does call
qdev_set_parent_bus(), falling back to SysBus if unsupplied.

While at it, ensure that this new function uses the device_ prefix and
make the name more neutral in light of this semantic change.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Tested-by: Igor Mammedov <imammedo@redhat.com>