Using the --fork option, one can make qemu-nbd fork the worker process.
The original process will exit on error of the worker or once the worker
enters the main loop.
Suggested-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>