vfio/migration: Add load_device_config_state_start trace event

vfio/migration: Add load_device_config_state_start trace event

And rename existing load_device_config_state trace event to
load_device_config_state_end for consistency since it is triggered at the
end of loading of the VFIO device config state.

This way both the start and end points of particular device config
loading operation (a long, BQL-serialized operation) are known.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/qemu-devel/1b6c5a2097e64c272eb7e53f9e4cca4b79581b38.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>