qdev: Remove DeviceClass::init() and ::exit()

qdev: Remove DeviceClass::init() and ::exit()

Since no devices use it, we can safely remove it.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180419212727.26095-5-f4bug@amsat.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Removal of DeviceClass::init() moved from previous patch, missing
documentation updates supplied]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180528144509.15812-5-armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>