This is cleaner, because we do not need to close the block device when
there is an error opening /dev/nbdX.  It was done this way only to
print errors before daemonizing.

At the same time, use atexit to ensure that the block device is closed
whenever we exit.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>