qemu_conf: Fix broken logic for adding passthrough iscsi lun

Following XML would fail :

    <disk type='network' device='lun'>
      <driver name='qemu' type='raw'/>
      <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi/1'>
        <host name='example.com' port='3260'/>
      </source>
      <target dev='sda' bus='scsi'/>
    </disk>

With the message:

error: Failed to start domain iscsilun
error: Unable to get device ID 'iqn.2013-07.com.example:iscsi/1': No such fi

Cause was commit id '1f49b05a' which added 'virDomainDiskSourceIsBlockType'

src/qemu/qemu_conf.c

@@ -1065,7 +1065,7 @@ qemuSetUnprivSGIO(virDomainDeviceDefPtr dev)
         disk = dev->data.disk;
 
         if (disk->device != VIR_DOMAIN_DISK_DEVICE_LUN ||
-            virDomainDiskSourceIsBlockType(disk))
+            !virDomainDiskSourceIsBlockType(disk))
             return 0;
 
         path = disk->src;