cli: Add --disk driver.discard_no_unref=on|off

E.g.
  virt-install \
  ... \
  --disk /tmp/disk0.qcow2,size=16,driver.type=qcow2,driver.discard=unmap,\
  driver.discard_no_unref=on

It results in the following domain xml:
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' discard='unmap' discard_no_unref='on'/>
      <source file='/tmp/disk0.qcow2'/>
      <target dev='vda' bus='virtio'/>
    </disk>

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Lin Ma <lma@suse.de>

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

@@ -445,6 +445,11 @@
       <source file="/tmp/disk1.qcow2"/>
       <target dev="vdv" bus="virtio"/>
     </disk>
+    <disk type="file" device="disk">
+      <driver name="qemu" type="qcow2" discard="unmap" discard_no_unref="on"/>
+      <source file="/tmp/disk2.qcow2"/>
+      <target dev="vdw" bus="virtio"/>
+    </disk>
     <controller type="usb" index="0" model="ich9-ehci1">
       <address type="pci" domain="0" bus="0" slot="4" function="7"/>
     </controller>

tests/test_cli.py

@@ -639,6 +639,7 @@ source.reservations.managed=no,source.reservations.source.type=unix,source.reser
 --disk source.dir=/
 --disk type=nvme,source.type=pci,source.managed=no,source.namespace=2,source.address.domain=0x0001,source.address.bus=0x02,source.address.slot=0x00,source.address.function=0x0
 --disk /tmp/disk1.qcow2,size=16,driver.type=qcow2,driver.metadata_cache.max_size=2048,driver.metadata_cache.max_size.unit=KiB
+--disk /tmp/disk2.qcow2,size=16,driver.type=qcow2,driver.discard=unmap,driver.discard_no_unref=on
 
 
 --network user,mac=12:34:56:78:11:22,portgroup=foo,link_state=down,rom_bar=on,rom_file=/tmp/foo