Block jobs & NBD patches
v2: - add arguments to QEMUMachine constructor in test, to make it work
on arm in gitlab pipeline
- use bdrv_inc_in_flight() / bdrv_dec_in_flight() instead of direct
manipulation with bs->in_flight
- add new options for copy-before-write filter
- new trace points for NBD
- prefer unsigned type for some 'in_flight' fields