vfio: Make vfio_devices_dma_logging_start() return bool

vfio: Make vfio_devices_dma_logging_start() return bool

Since vfio_devices_dma_logging_start() takes an 'Error **' argument,
best practices suggest to return a bool. See the api/error.h Rules
section. It will simplify potential changes coming after.

vfio_container_set_dirty_page_tracking() could be modified in the same
way but the errno value can be saved in the migration stream when
called from vfio_listener_log_global_stop().

Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>