virt-install: Add support for streams attribute for 'virtio' sound card

E.g.
    virt-install \
    ... \
    --sound model=virtio,streams=4

  It results in the following domain xml:
  <sound model="virtio" streams="4"/>

Signed-off-by: Lin Ma <lma@suse.de>

tests/data/cli/compare/virt-install-many-devices.xml

@@ -807,6 +807,7 @@
       <codec type="output"/>
     </sound>
     <sound model="usb" multichannel="yes"/>
+    <sound model="virtio" streams="4"/>
     <audio type="spice" id="1"/>
     <audio type="pulseaudio" id="2"/>
     <video>

tests/test_cli.py

@@ -750,6 +750,7 @@ source.reservations.managed=no,source.reservations.source.type=unix,source.reser
 --sound ac97
 --sound codec0.type=micro,codec1.type=duplex,codec2.type=output
 --sound model=usb,multichannel=yes
+--sound model=virtio,streams=4
 
 
 --audio id=1,type=spice

virtinst/cli.py

@@ -4775,6 +4775,7 @@ class ParserSound(VirtCLIParser):
         cls.add_arg("codec[0-9]*.type", "type",
                     find_inst_cb=cls.codec_find_inst_cb)
         cls.add_arg("multichannel", "multichannel")
+        cls.add_arg("streams", "streams")
 
 
 class ParserAudio(VirtCLIParser):

virtinst/devices/sound.py

@@ -24,6 +24,7 @@ class DeviceSound(Device):
     codecs = XMLChildProperty(_Codec)
     audio_id = XMLProperty("./audio/@id")
     multichannel = XMLProperty("./@multichannel", is_yesno=True)
+    streams = XMLProperty("./@streams")
 
 
     ##################