qemu: Don't try to start NBD server twice

qemu: Don't try to start NBD server twice

In one of recent patches the way that we start NBD server for
incoming migration was reworked (v6.8.0-rc1~298). A new boolean
was introduced that tracks whether the NBD server was started so
that we don't start it twice nor record in the port in the port
allocator twice. Well, this idea is good, but in the
implementation the boolean is never set, so we are reserving the
port twice and would be starting the NBD server twice too if it
wasn't for port reservation fail.

Fixes: e74d627bb3bc2684cbe3edc1e2f7cc745b4e1ff3
Reported-by: Vjaceslavs Klimovs <vklimovs@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>