qdev: HotplugHandler: Rename unplug callback to unplug_request

qdev: HotplugHandler: Rename unplug callback to unplug_request

'HotplugHandler.unplug' callback is currently used as async
call to issue unplug request for device that implements it.
Renaming 'unplug' callback to 'unplug_request' should help to
avoid confusion about what callback does and would allow to
introduce 'unplug' callback that would perform actual device
removal when guest is ready for it.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>