qemu: Release nbd port from migrationPorts instead of remotePorts

qemu: Release nbd port from migrationPorts instead of remotePorts

commit 3e1e16aa8d4238241a1806cb9bdb3b9ad60db777 (Use a port from the
migration range for NBD as well) changed ndb port allocation from
remotePorts to migrationPorts, but did not change the port releasing
process, which makes an error when migrating several times (above 64):
error: internal error: Unable to find an unused port in range
'migration' (49152-49215)

https://bugzilla.redhat.com/show_bug.cgi?id=1159245

Signed-off-by: Weiwei Li <nuonuoli@tencent.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>