qemu: implement QEMU NBD source reconnect delay attribute

qemu: implement QEMU NBD source reconnect delay attribute

Currently it's only possible to set this parameter during domain
creation via QEMU commandline passthrough feature.
With the new delay attribute it's also possible to set this
parameter if you want to attach a new NBD disk
using "virsh attach-device domain device.xml" e.g.:

  <disk type='network' device='disk'>
    <driver name='qemu' type='raw'/>
    <source protocol='nbd' name='foo'>
      <host name='example.org' port='6000'/>
      <reconnect delay='10'/>
    </source>
    <target dev='vdb' bus='virtio'/>
  </disk>

Signed-off-by: Christian Nautze <christian.nautze@exoscale.ch>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>